This just needs to happen. Have the crossover be about the 12 sealed weapons and about how Gilgamesh is actually the Guardian of those weapons, but during his travels through the interdimensional rift, he got corrupted.
Have the 12 sealed weapons previous owners be legendary characters from soul Calibur.
Hailing from a distant star, one of them arrive to Etheirys to attempt to find one of the missing weapons to prevent some evil guy (void guy from whatever ff game) from getting it.