Perhaps you could check out the game Renryuu: Ascension. There's a instance in the lower levels of the Spire of Courage. In which a "raid" as is common in a MMO can found in a single player game and yet, possibly be tweaked and perhaps become even better? But, I guess I will have to check out this next game of yours as well, when able to, that is.
As for mechanics go though. There's a large variety of them. They go from dungeon crawling, to events, to boss fights. You have one that frustrated myself to a point that I had to look up the fight with Ginasta. That fight I tried to go all out offensive lol that was a fail.. (currently near the end of Zirantia, I think? Currently at the end of the 3rd ruin). As for the open world concept goes though. DoE is one example to go outside of the more 3D aspect of some of the games on here(despite the 3D ones cause high fps issues, even on a high end gaming pc).
As for Patreon goes, you just might be the one I go for first. I was anticipating with going with the DoE creator since the perk of having someone's own custom character and custom room added?? It sounded very tempting to go with. Although, the character I would of chosen wouldn't fit that world at all. But, here in TLS? It would, especially with all the furries added to this game. But, then again the picture icon I chose to use would be a give away on what that would be. 😉