A single, square patch with custom logo/insignia easily added via JPEG file, in similar fashion as Half-Life had their spray logos in MP, to either show-off personal attitude or gaming clan affiliation.
More sophisticated patch system with in-game supplied set of patches, something like in-game rank, critical missions finished or usual team role would be pie in the sky, then again, there are almost no games that would take any serious effort into that...
But at least a slot for a single, custom (player supplied), personal patch would be great.