My picks:
Stanwick piddly many think that jayce is viktors arch enemy,but thats not really true its this guy,he stole viktors research and made urgot.He could be some kind of robot master or maybe use a power armor ala iron man
Boram darkwill and his sons
Seriously Borams end was extremely lame and kinda anticlimatic,I mean he is THE guy who shaped noxus into the nation it is today and we never even got to see boram.Sure Boram is officially dead,but if I remember correctly his body was supposedly burned(so it could have been faked) and he was known to have necromantic powers,so it wouldn't be off if he returned as an undead(His son kieran would be perfect as the "headless-horseman" kind of guy)
The ruined King
Mostly because I finally want to see the guy and don't want him to end up like Boram.I dunno what he could do ingame,yorick is already the necromancer guy.
General du Couteau
He is one of the most prominent background characters,but we never got to see him.The big problem with him would be how they could differentiate him from talon or katarina since he is known as a blade master and assassin,which is already their thing.
Kassadins daughter
Just because I want to know what happened to her
The other 4 darkin
I am pretty sure they'll all be playable at one point,but I'd really like to see more of the darkin simply because we know almost nothing about them.