You know how some Boss Mode levels don't allow certain hero types? Like nothing but Fire, only Warriors, no Light heroes, etc?
I think some lore-based events that wouldn't allow certain heroes would be interesting. Like perhaps story wise, there would be a war between two heroes, let's say... Malice and Emily, and for the first half you can choose either and play as either, but for the second half there's a vote for everyone's favorite, and the least favorite "dies" so you can't play as them in the event dungeons for the remainder of the event, and the winner gains a temporary buff.
There's of course much more possibility. Perhaps a hero can simply be injured in a battle with their rival, and gain a visual update and new abilities. It would be a fun way to rework older heroes.
Favorite hero: Indigo
Currently: Trying to get as many runes done as possible. Takes a lot of money, man!