Not joining...just neutral and not necesserely demigods. Yes, this could work, like in DOTA, where there were neutral heroes on the map, and you could kill them if you wanted, for some item or gold. And then they will respawn and stay at their place for the rest of the game. But for this to work, we will need new maps, designed with them in mind...
Anyway, it's wishful thinking, i doubt they will implement this (or other things that take too much time to make)....