I have a handful of mostly-disconnected comments, and unfortunately not much in the way of solutions, but maybe some of you who have been at this longer than I have can make something from it heh.
One of the things Fantasy Flight Games (FFG) uses for their organized play is the concept of a restricted list - cards you can have in your deck, but you can only choose one of those cards to use. It's basically a way for them to retroactively limit cards that they realize should never have been printed without making them an auto-include in every deck. I wonder if just enforcing something like this at various levels is a quick-and-easy way to reign in some power creep? I saw someone mention limiting the number of UR tokens in normal, or legendaries in hardcore, as a way to keep things in check a bit - though I wonder if that causes big problems with people having their treasure enhancers turned off by being forced to a lower difficulty. Maybe another option would be to cap the number of slotless items that can be worn?
One of the oft-forgotten guidelines to engineering and design is that more complexity is best avoided. If there's a way to solve your problem by taking away or simplifying rules rather than adding new ones, that is often the more elegant solution. Easier said than done and I have no great ideas on how to do that, but it's something to keep in mind if anyone comes up with a suggestion for that. (At the very least, I'd look at that as something going forward - a request to avoid adding new slots to fill).
As an observation - as a new player at GenCon 2018, one of the obvious problems I saw with the token system in general was that there is a pay-to-win element to the game, and that power creep means the barrier to entry is only going to get higher. Whether that problem is real or perceived is another matter entirely - we ended up running a sealed quest with no problems on normal, then the rest of our runs were relatively smooth as well. I've heard from a few friends that the 2018 dungeons were a jump in difficulty over previous years, so if our group of <40 tokens to our name - mostly commons at that - was able to stumble through with just a couple people in better gear, maybe there's less paywall than it seems.
There's something to be said for the newer players' unintentional slowplaying of a boss encounter as well. Two rounds of attacks plus push damage at low difficutlies may be comparable to 3 rounds of getting hit by the boss and has the same effect of getting to the next room to continue the adventure. So there is a self-correcting gimmick that newer players actually end up in a situation where their damage output may be straight-up irrelevant. Some of us got talked into a hardcore run at PAX this year and we never killed a monster, but the only players to die got auto-killed by Byfrost rather than damage.
I feel like a lot of the need to create and enforce balance comes from co-op board games and video games. There's a goal of saying "I finished this and accomplished it fair and square." If you can accomplish it by just throwing a wallet at it, that cheapens it a bit - but the vagueness of what constitutes "enough" gear is problematic. Where am I bringing the party down? Where am I trivializing this for someone else? I would personally advocate stat caps, down-sync'ing, or some system to that effect, but in their absence I would strongly request clearer guidance on recommended gear levels.
… Like I said, it's an eclectic collection of thoughts rather than any real solutions or offers, but hopefully that's useful. Thanks for reading my post.