Can't be GTA6 since they only work in one game at a time and that game is RDR2.
I don't think MS would give their big E3 reveal away.
With the leaks, it makes sense if it's Soul Calibur 6.
I can't see it being MGS6 considering they have been working on metal gear survive. Secondly, Keighley worships Kojima. The likely hood he would give Konami the stage to show off MGS6 is zero.