Imagining that Josh Brolin’s Thanos has already thrown down with Wolverine is certainly fun, but with all the threats and opponents that he’s combated over his long lifetime, there’s no shortage of possibilities of who or hat could have clawed Thanos’ face like that.
If Thanos is anything like his comic book counterpart, he’s been around a long time and taken part in a lot of battles.
Hell, if his own claws were outfitted with vibranium or a metal similar to adamantium in durability, Rocket Raccoon could have scarred Thanos’ face.
One doesn’t become one of the most feared warlords in the galaxy by sitting on the sidelines and letting flunkies handle all your battles.