Since there are two slots listed as 'coming soon' in the single player campaign, it's likely safe to believe one will be a new faction entirely, and the last will possibly be another new faction or a 'play with your fave faction' type situation. Either way, we have Demons, why not Angels? There regular units could be things like Archangels, Cherubim, Seraphim, etc, with heralds could be like any of the big angels like Michael, Raphael, Gabriel, etc, and of course other heroes could be things like Metatron, Angel of Death, etc etc etc. and spells like divine intervention, holy light, or whatnot stuff like that.
I don't know, think it would be good and give a lot of people a probable new favourite faction, I mean sure lots of people like the demons, why not the angels?