I don't see why he needs any weakness.
Thor can be beaten in combat by the likes of Hulk, Hercules, Beta Ray Bill, Kurse, Juggernaut, Gladiator, Crusader, Red Hulk, Silver Surfer, Midgard Serpent, Surtur, Thanos, Ultron...
Moondragon and Lorelei (with the help of Loki) were able to control his mind/emotions.
Hela was able to curse him, making his bones brittle.
Criminal master minds like Doctor Doom, Kang and Magneto certainly know how to deal with Thor.
So, Thor isn't overpowerd. There are a lot of enemies in the MU who are very dangerous for him. I don't see why he needs a weakness. Hercules is Thor's equal and doesn't have a weakness either.