Is it possible to play as the Hero faction in a skirmish or multiplayer match? That would be very interesting. Maybe implement a game mode that involves Player controlled empire faction. That sounds like a good idea!
It's more a case of "if you corrupt Empire units while playing as an Underlord, they will actually be able to work in your rooms" and the idea of having "Corrupted Gateways" in some skirmish maps that attract Empire units. The first will almost definitely be in - anything regarding the latter is still very much under internal debate. We can't say where we're headed with in this direction at this point in time.
I know this would be a design extravagance, but it's a shame that when upon converting hero units, they don't take a more 'evil' guise, e.g. the dryad going on flames or resembling a haunted tree being. It may out-compete the demons at their own game, though, and I guess some people want to pull the actual heroes by their puppet strings. It still would be a fun optional mechanic - re-training the now converted heroes to be evil.