MArcomage

This is a really funny card. I can't really decide whether or not it's overpowered. Obviously in a construction deck, this is a suberb card once you've got a rare in hand. Even if you can't get one, it's good as a finisher or semi-finisher when your tower is getting close to max. It probably has some good applications in a plague-y deck especially with Mercenaries, but I'm not in a good position to speculate on that side of it.

I don't know if I like the idea of a common card being so swingy depending on whether or not you have a rare. Maybe it would be better to buff it and make it an uncommon?

That would destroy the very purpose. :)
And getting rare is easy early in game, it's all about facilities.
Only thing I forgot with all the recent mechanics change talk: would it raise facility via keyword BEFORE or AFTER card effect?

(So it does not raise and lower the same facility)

Keyword and card effect are both executed separately and after both are the game limits applied. So it's possible to gain 1 facility with this card.

Great
Fun aspect would be to summon it with summon master

Tower +2?