I think I remember a post from a long time ago that the Archmage's are planned to be named mages from the lore of the game. Hopefully that means there will be a decent amount of lore by then...
I think that an Archmage should have increased spellbook points, health, and channeling. They could be the only mages that start with armor too.
I think extra actions makes sense, and they could either have a second quickcast marker, or a second action marker, maybe depending on the Archamge. Or there could be a new full cast only marker, that you could use to either do an extra quick action or an extra full cast, but you couldn't use it to move.
I also think it could be fun if the Archmages had some kind of weakness, they get their extra powers because of some object that they enchanted, so if your team destroys that then the Archmage becomes weaker.
I never really got into Magic, so I'm not sure how the Archenemy thing works, as opposed to an Archmage.