Support that makes enemies take more damage through debuffs
passive
Support that increases mana/energy sustain or reduces allies cooldowns
Old
(got removed because it's too strong) / not very likely as every champion is balanced around their CD (Zilean has a refresh and his kit is built around it)
mage that has spells that can crit e.g cdr gives mage crit but mage does not benefit from cdr
spells deal extra damage under certain conditions
Unlike AA spells can't balance out "real" crit damage, since they lack the balance over time (Criting with 1 out of three attacks isn't a big deal, but critting with a nuke spell instead of your two other spells is something completly different)
Chariot archer or centaur archer
Since we already have Kled and Heca as mounted warrior and centaur (Chariot would have a pretty big model unless another yordle) this is very unlikely
AP riven? ( DOTA invoker)
Invoker isn't easily balanced in the context of this game ending up as either OP or UP
battlecry based champion (guildwars paragon)
AOE buffs /debuffs aren't very interesting designs as they provide little visual and emotional impact and usually have low risk high reward.
Ability thief(rogue from x men)
Since LoL champions are built around their entire kit having a champion steal one ability isn't balanceable
(having Rek'Sai Ult vs getting BC hook for example)
Mobility based poke mage
is a caster. Most mages with high mobility usually end up as assassins
etc.
AP rifleman
AP "Marksmen"
AP Rifle

Prankster
?