Just a really random idea of mines I thought up of. There's actually two versions I thought up of for this idea, actually. I'm not limiting these to arcade mode to "revive arcade mode" or whatever, I just don't think something like this would fit in missions.

-Ver. #1-
For levels that are randomly generated, make it so there's a small chance of having a mandatory battle room that's sorta shaped up specifically for some sorta miniboss. When the party all steps the party button, then the fight starts against the miniboss (or even minibosses, if you can safely spawn two of these without messing up any sorta gimmick they would have and/or without making it too hard). The miniboss(es) doesn't have to be anything too big of a fight-just something that's at least stronger than a guardian class enemy (e.x. Trojans, Giant Lichen Colonies, etc.) and has some sorta gimmick to it. Some underlings may also be spawned with this enemy/miniboss(es). The miniboss(es) spawned will fit with the theme of the level (or stratum). Again, nothing too major (like the actual bosses) that would actually require you to have a long or scenic fight with it or have a mission/level specifically dedicated to it or anything like that. These enemies/minibosses will never spawn anywhere else other than in their own specific battle rooms. Depending on the rarity of these battle rooms appearing, a rare drop may or may not be viable. EDIT: Alternatively, make the miniboss(es) really hard and limit it to elite mode, or make elite mode have its own set of minibosses that are a lot harder than the normal ones that would spawn in these battle rooms. Don't know how well of an idea that would be though.

-Ver. #2-
This is more boss-like than the version stated above. Sorta like danger rooms, have a small chance of optional rooms appearing in randomly generated levels, where they would fight a unique boss monster or spawn a number of certain type unique enemies/minibosses that doesn't spawn anywhere else. These rooms will also be specifically made for fit whatever gimmick the boss monster/minibosses have or just to fit the theme of the boss/minibosses. Depending on how rare and how hard these will be and whether they will be placed behind an energy gate or not, there may be unique rewards or tokens or rare drops or none at all, but the room filled with health boxes and treasure boxes should be mandatory at the end. I would suggest that it fits with the theme of the level or stratum, but I don't think danger rooms actually do that, so... I don't know.

I don't have any specific bosses/minibosses/unique monsters and rewards thought out for this idea. If people want me to, then I can make some. For now, I'm just going to throw this idea out there and see if people like it or not. I don't really care too much whether or not people likes this idea, it's just something that I thought up of that I thought might be a good addition to the game.