I kinda like the horseman becoming the level 5 unit
In fact, it'S the one that make sense since it'S the only one which got something more then the other pedestrian units, since it's on horse.
I think making the horseman the ultimate dragon slayer should be in order!
But I think everyone miss the fact that there is a level 5 unit for human, it's the Phenix you get from the level 3 spell!
Another option should be making the Royal griffin a level 5 unit. The only thing I'd tweak is the summon ability, making it reloadable with time like the summon ability of the Royal thorn (maybe that's way to good tough...!)
I think it's the easiest one to implement since Royla Griffin are not sold that often compare to horseman/knight/ paladin
PS Is it me or there's a place for a lower version of the ArchMage... the Mage! Should have made ArchMAge little bit tougher nut more expansive and put a level 2 Mage