How do you plan to play-balance user-created content?  Will object (unit, creature, spell, etc.) creation be on some sort of points-based system?
If so, I have to recommend using some sort of non-linear sliding scale.  In most games, specialization is king.  It'd be cool if specialized things had their place, but came at some sort of cost.
Just my uninformed 2 coppers, of course.  