League doesn't have a Warlock ala Gul'dan, unless I'm mistaken
Swain is the closest we have, but doesn't quite have "Warlock" in his design. Besides, he's slated for a VGU, AFAIK.
Gul'dan is one of my favorites right now in Heroes of the Storm because of his high damage, but also because his Life Tap mechanic is so unique for a MOBA. You don't regenerate mana normally, but pay a portion of your health to recover it. I imagine this wouldn't be doable in the exact same way in League due to the tech limitations (D key is reserved for Summoner Spells, for one), but it'd be cool to have Warlock mechanics in LoL.
Not to mention that Runeterra could always use a Chaos Mage.