I just thought of the most epic crossover game ever. Doom + Wolfenstein + Serious Sam + Duke Nukem. Blazco gunning down the hordes of Hell, Doomslayer ripping Nazis in half with his bare hands, all while Duke and Sam take turns spouting one-liners and taking down every alien in their path. Bonus points if the hypothetical maker could find a way to end up revealing to Blazco that Doomslayer is his descendant.
"Come get some, you Nazi motherfuckers!"