those are the main 2 games I play, so if I worked on both I would have secretly added some skyrim type characters with all their skills, and had it all worked into the BF3 code with all the required damage animation...programing...everything that was required was built into all BF3, and assume it was hidden well enough that it would take a long long time to be noticed
and then just every now and again I alone would turn into a skyrim character in a BF3 server, kill a few people with flaming swords and atronachs, then turn invisble run off and leave the server
no one would believe my victims that a skyrim style player appeared and killed them
I would become a BF3 myth, but be real to my occasional victims

...