I have an idea that I believe would very much help keep the high level player base or even bring older ones back. I think introducing player made levels would be the answer. If drop balancing issues are a worry then you can easily just make it so only combat consumables and hearts drop from enemies; because I am very sure experienced players would just love to have new, ever-changing combat content they can do.
From what I have noticed, everything in the game seems to be grid-based, so you can make it kind of like the construction mode in guildhalls. Just have it so you can place hazards such as spike and poison traps, enemies, blocks, switches, triggers, gates, and all the other things that go into levels that could be escaping my mind at the moment. Have prize boxes be place-able, but only have them drop combat consumables such as vials and pills.
Being able to place bosses would be super cool too, so people could make their own boss rush modes or even just silly, fun, or challenging levels where you do things like fight 3 Vanaduke’s at once, but I could understand if it doesn’t work out because you place bosses that are used to the same confined areas into places that can be freely molded.
But getting back to the point: you can make it so you have to beat the level yourself before releasing it as a sort of content control, and you are set! There could be a ranking system where players rank their favorite player-made levels, as well as a “weekly featured levels” for the most upvoted levels of the week, and a legacy ranking where it has the top most rated player made levels of all time.
To make the levels easier for players to sort out you can add a search option by level name, player name, or even a tag system where players can put tags related to their levels so they show up in a search.
If you want to make it lore-friendly, you can have Kozma contact the player and inform them they were able to recover the technology of simulation rooms from the wreckage of the skylark or something along those lines, like what was done with battle sprites.
Now I will not ignore the fact that some people can be rather malicious, especially when they have the ability to draw or make things freely. To combat this, you can do something like lock the content behind a certain rank and add a report feature for levels.
Well, this is just kind of an idea I had when I was sitting around in Haven just hoping there was a way to mix up some of the levels other than waiting for random gates to appear. I strongly believe players both new and old would welcome this, especially those dedicated players that feel like they have reasonably done everything in the game there is to do and just want to have some dumb fun, or face a true challenge. To me this would be a dream come true, and I know that it also would to many others out there.