Leveling:
My desire is to make leveling occur at the exact same pace as present, while simplifying it's tracking and still rewarding good play.
So let's do some math (I promise your players will be doing less of it, and you won't have to track XP per monster at the end of modules/sessions). In recent seasons, the APL of modules has, for the most part, been standardized around levels 3/8/13/18, or at least pretty close. If you assume minimum XP in each of those modules, you'll level at a rate equal to that described in the rules document.
However, minimum XP isn't what we get most of the time. So we want DMs to have the flexibility to award good play. Currently, there isn't much adjustment they can give, it's nearly all based on combat. So give the DMs that power. Allow them to adjust the gain up to an amount equal to what Max XP was under the prior system. Conveniently, that's pretty much 30% more than minimum XP was.
So let a DM award up to 1.3 checkpoints per hour if the players do well. Yes, it requires trust in DMs, but this is an honor system to begin with, and furthermore, some DMs get tired of doing the math now anyways and pick a spot somewhere where they think is right. (Is that right, no, but it happens)
Now you're empowering DMs to reward creative play (allowing focus on the story, not the math), and allowing RP and exploration to take a bigger percentage of the stage, not just combat.
So now, the math becomes 'Well, we played 2 hours. I can award you from 2 to 2.6 points. You guys did a pretty great job, but you hit the trap pretty badly so how about 2.5 per person' Player writes down 2.5, adds it to their existing total, let's say 35, then checks a chart just like before, sees, oh, I'm at a total of 37.5 now, that's higher than 36, I level up to 6th. The DM didn't have to add up monsters and divide by players, plus add in the 'all players get X XP awards', see if that's between min and max or not, tell the players a 3-5 digit number of XP, slightly harder math to add, still have to check a chart to see if you level.
in addition, since the leveling is not flat across levels in a tier, you get to spend more time at the higher end of the tier, which allows you to get adjusted to your increased power better and to feel stronger for a longer period of time. This progression also /exactly/ matches what we have now in terms of time played translating to levels gained, so there's no concern about outleveling a set of modules in a tier for one season.
Note on the exception - Yes, there are some modules off from the 'common' APLs of 3/8/13/18, but the only one to produce a drastic disparity in terms of XP is modules for APL 1. So for those APL 1 modules, we award half a checkpoint per hour so you don't outlevel that set.
Gold
Why do we keep it the same? Well, we have 3 options - Reset everyone onto a new system, Go to a new system but don't reset existing players, or keep it the same. Let's take a look at those
Reset everyone:
If you reset everyone, you have 2 problems. 1 - People who already spent their money would have an extra benefit. Say you built a golem. We're not taking the golem away, so even if we set your gold to 0, you may have received more benefit from gold than is possible under this new system. That's unfair. On the other end, if you didn't spend your gold, and we reset you down to some number, we're either penalizing you, or incentivizing you to 'purchase' a boatload of mundane gear before the switch and sell it after.
If you go to a new system, but don't reset existing players, you hit many of the same issues. New players have a different amount than existing players. This is /very/ bad when you open up purchase of consumable items with just gold as a currency. An existing player could buy hundreds of scrolls, allowing them to have much more utility than a new one. You could use those items to avoid the need for rests, to handle issues that would normally require you to use a spell slot on them, and make other players feel less-important since they don't have the same abilities.
Another issue of a fixed gold system is that it removes many of the incentives for certain kinds of play, and neutrers choices that are part of existing content. So, Mr. Tavern Owner, you say you want us to do this task for you? Why should we? "I'll give you gold" is the most common reply. Without that being an actual thing, it just seems empty. Why look around in tombs and potentially trapped chests and hallways if your gold and treasure points just come from time. If the only thing you can get is consumables, and you can just /buy/ many of those, you're risking potential greater loss to get something lesser. Worse, a number of important decisions in modules are based around gold. In one module, you have the opportunity to take 'cursed' gold. Why would you do that if the gold doesn't have meaning? Sure, some might, but certainly not at the same rate as before. Or a module wandering through a catacombs, where you're told not to touch anything, or another module where you're in a room of illusory gold that causes problems if you touch it. Or the yellow mold that looks like gold... basically, without gold, many scenarios that already exist fall apart. Looting gold is too integral a part of DND and adventuring to handwave it.
Lifestyle
This seems like an area where the XGE rules did something right. Handwaving downtime gold costs is a good thing, the adding or subtracting 1-10 gold per day is such minutae that over the course of a campaign it doesn't matter much. I came up with the early advancement numbers by multiplying the number of downtime days in a tier by the extra gold cost difference between the lifestyles.
Consumable magic item purchase
I like XGEs system, but I think it's a little too easy with current gold values, as I suggest preserving. At a moderate level, you can now walk around with all the out of combat scroll you want. It makes it much easier to conserve your spell slot resources if you're never using them out of combat. You could boost the cost quite a bit, but that makes them out of reach for earlier tiers but not solving the issue at higher ones.
What people don't have at any tier is massive surplusses of downtime. So stick a downtime cost on top of the gold cost.
The items priced at under 75 should take no DT
The items priced at 75 to 150 should take 1 DT
The items priced at 151 to 500 should take 2 DT
The items priced at 501 and up should take 4 DT
Knock Invisibility potions down to 600. 1/3 classes can cast invisibility, meaning for out of combat uses, the 150 gp scroll is plenty. Just carry ones for yourself and distribute to the party caster as needed. In combat, the potion has benefits because it's a touch spell and you don't have to convince the caster to use his turn for your benefit. But not 4850 gold of benefit.
This means for 2 hours of gameplay, if you choose to use your downtime in this way, you can get:
Any number of cantrip scrolls and standard healing potions
Up to 5 scrolls of 1st/2nd level, potions of climbing, animal friendship, greater healing or water breathing
2.5 scrolls of 3rd/4th level or superior healing
1.25 scrolls of 5th level, potions of supreme healing or potions of invisibility
Magic items, found
I like treasure points. They prevent item sniping, this is a great thing. But they also get rid of serendipity, and excitement. They make it possible to engineer a build from 1-20 precisely, and likely will promote something like netdecking, which I don't think is great.
As currently written, they also highly discourage picking consumables (Doubly so when you can buy plate, sell it, then buy the consumables cheaper with gold). They also skew value of items greatly. Why would I pick a potion of flying from D, when I can get a broom of flying from F for the same cost and a tier earlier? Seems awful inefficient to get a potion of storm giant strength for 8 points, versus a belt for 12.
In the existing system, players at small tables are disproportionately rewarded, as well. If Jim always plays with his friends at a table of 3, and Steve plays at a table of 6, Jim will get twice the items in his career compared to Steve.
So two things - you promote equity when using this system, as what you essentially do is make it so each player plays at the equivalent of a 5 person table and picks 1/5 of the items that appear. No more skewing between someone who gets a bunch because they're at a table of 3 and someone at a table of 7 who gets very little. Also, you still make treasure mean something. I still have a reason to search more for loot, as getting it means more than just unlocking an item to purchase, it's also the currency with which I purchase it.
should be a story award to not force someone to purchase the item. I don't think they're powerful enough to ban.
I also selected the 1/2/4/8 scale because I think the 8/10/10/12 scale highly encourages picking the biggest, strongest items. Why would I pick a +2 Int Tome and 2 points versus a Staff of the Magi? But when it's a +2 Con and a +2 Int tome both, it makes you stop a little more. Also, with the disparity being greater, it isn't as big a deal to save up points across a tier. So you pick nothing at tier 1 to go for a legendary when you hit T4... well, congrats, you've saved up around 9 points, out of the 40 you need. You won't even hit 40 until around level 10. So sure, save them, it won't hurt anyone else, and in the end, you'll still be sacrificing 8 uncommon items or 4 rare items for that one legendary. Seems a fair trade to me. Plus since you can't buy them until you find them, not a huge deal.
I agree that the McGuffins should be story awards, but I don't agree that they're too problematic such that they need to be banned, allow purchase of them like any other legendary. However, the issue with them is that you can trade them. You can trade a Sunsword for a Staff of the Magi. So the solution is to get rid of the Sunsword? No, that's fine at Tier 2. You just ban the trade of an item outside your tier until you get to that tier.
If you go forward with the proposed problematic magic item list, you make it worse! Why? Let's say you're a high T1/low T2 with a Sunsword or Dawnbringer. Perfectly legitimate time to have gotten those items. You have a T4 character (or a friend with a T4 character) with, say, an Ioun Stone of Regeneration. Any non-'problematic' tradable legendary, really. Before August 30th, you trade the two items, so the T4 has the 'problematic' sword. On Aug 30, it becomes 12 treasure points. You pick +3 armor, or a Cloak of Invisibility or a Staff of the Magi. You now trade that back to the T1/T2.
You've now turned the 'problematic' sword into a Staff of the Magi in T2. I assure you, someone dropping 7 level 7 fireballs and absorbing single target spells is much worse than a sword that glows and does radiant damage.
Solution? Most of the items on that list aren't problematic. Remove them from the list. Then, require trading to only be legal if the item traded is tier appropriate for you. So you can't trade a legendary you acquire until Tier 4 (VR for T3, R for T2). So when you get to T4 and trade that Sunsword for a Staff then, well, it's T4, so it shouldn't be that much of an issue (even if SoTM is one of the strongest possible items out there)
DM Rewards
I based these on average gold from modules, and now DMing any tier is just as worthy as DMing another tier. I tried converting the quests appropriately as well.