I agree with ya' Greg, on all but one point: that hero modes should be able to be completed whilst in villain mode play.
This, in my opinion, is unnecessary and makes those modes clunky with too much going on. Villain modes should be focused, as they are now, except for the brotherhood and hellfire MBs, which I think should still stack for hits.
I've explained this before, but I'll lay it out again: it makes complete sense for certain completed (solid) hero mode combinations to make certain villain modes easier.
We have to bring more story into this pin through gameplay, allthewhile insentivising players to make strategic choices in order to give another layer of depth and strategy to it. So here's the scenario as an example: let's say that Sabertooth is "weak" to the combination of Wolvie and Storm. If you start Sabertooth mode after fully completing at least both Wolvie and Storm, Sabertooth takes 1/2 as many shots to beat, but even if you have just Storm completed, and Wolvie started, but unsuccessfully completed, you fight Sabertooth as normal. Each villain has a specific set if completed Heroes that they need to get the bonus during fighting them, with the Mag, Brotherhood and Hellfire MBs benefit according to the total number of heroes completed up to that point.
This brings more story and strategy, makes total sense, and gives you a reason to pick certain villains to fight during play.
Oh, and Stern, please give us jackpot callouts!!!
...rant over.