I think it would be nice to get another solo arena similar to maelstrom (of course there could be solo or 2 / 4 player versions with adjusted difficulty)...the reason why i think it would be interesting for solo - one of the things that is frustrating about maelstrom is that its always the same and that becomes soooooooo boring and tiresome to do... with multiplayer arenas this gets compensated by the group aspect which makes everything a bit more fun...
So my idea for a solo arena would be a dungeon that you have to go through with a certain target, the dungeon contains mobs, traps, bosses, maybe even some puzzles.
But here is my idea - the traps, mobs, bosses as well as their locations, and the treasures and their location as well as your target (finding a treasure, killing a boss or whatever) as well as the dungeon layout is randomized and gets changed every time you start a new run (inspired by labyrinth the board game).
Does that sound interesting to you or stupid?