Well I seem to running into a number of fairly ridiculous ones lately.
The Hallowed/Corrupted trophies from Terraria seem like they'll be quite a feat.
The collect all armor/items/titles trophies from Cross Edge.
Reverse Pirating trophies from Disgaea 3 among others.
The Borderlands trophies from the last DLC where you have to collect so many of certain items (The Lubricator, Bobble-Trap, and What a Party! particularly). That and the arena trophies from the Mad Moxie's DLC.
One Piece: Pirate Warriors the gain all skills and defeating over 100,000 enemies are both fairly ridiculous too, granted they have a kind of synergy I suppose.