So I've noticed one type of Playstyle missing from LoL atm...
The BattleMage!
BattleMages, those who give up some power in Magic and some power in Physical Prowess to be able to do both, often by using magic to increase their physical abilities or supplement them.
LoL doesn't really have this cool little Hybrid character that makes a really neat playstyle for those who like it. Players who like to be in the front but still cast some spells and deal damage, but have a sword.
And the best part is is that the BattleMage concept is so incredibly dynamic and flexible I personally would like to see Riot's take on it; Maybe a fighter who's melee weapon scales well with bot AP and AD, or maybe an Assassin who amplifies their damage with very good Single Target spells and weapon buffs. Or perhaps, even a character who embodies the flexibility of a BattleMage by allowing them to become either fighters/bruisers or Assassin/Mages depending on builds?
Personally, I would love to see this happen.

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. His ult has massive AD and AP scaling.