personally I've always wanted to see an engineer class. you can "construct" stuff for the battlefield like siege towers and healing tents, so we are halfway there. but obviously playing a engineer would be alot more fun if one could contruct a bastilla on the battlefield, think tf2 but medievil style. Balista balance wise should not be like playing a crossbowman, baslita would ideally be an anti-construction, anti-camp weapon but deadly vs a crowd to be sure ;)
but I never suggested this for reasons paul mentioned or didn't. plus I have new suggestion everytime I log in and try not to post them all.