Right now, characters seem tied to saved games.
It would be great, especially as dungeons are added (from the dev as dlc, or user created) to be able to have a roster of heroes from your other games to use to play through them.
Dungeons would need control settings to ensure balance -- for example, one dungeon might only be able to start with level 1 characters, or no characters over level 30, or one could require level 45 characters, etc.
However, i'd love to be able to take my guys on more missions.
Issues that may need to be dealt with:
1) can a hero go on multiple missions at the same time? i'd say yes... they don't need to be actually 'at the same time', even if you're playing them at the same time. also this prevents user created dungeons from perma-trapping your favorite hero with intentionally bad design.
2) what if someone creates an easy grinder dungeon? no problem. different people play different ways, so as long as all other dungeons have level requirements, the fact that someone cheesed a level 50 character doesn't really matter.
anyway, i think this would be a great addition to the RPG elements of the game, and easily let us select a squad for our newest dungeon exploration.
it also creates interesting dynamics - if you've been developing 8 characters (two runs through Grimlock) and you then play a different dungeon with 2 from each of those teams, once they finish you now have them leveled up to, say, 25, but your other 4 are still level 14 or so. Eventually you'll have a small army of different level'd characters to choose between for each new dungeon foray.
I'd love it.