Maybe IF hexblade gets added they could move him into that subclass, pact of the blade rapier would fit. If Mizora gets added she could be a weakened Devil warlock with Fiend?
http://dnd5e.wikidot.com/warlock:hexblade
Serious question: Can hexblade have pact with fiend? O_o
Since whole Wylls story seem to be wraped around that, and i kinda doubt that Larian will change it whole, just so Wyll can have specialisation that would fit his title.