Hellboy vs. the Slender Man. It would be totally awesome.
Berserk and Warhammer 40,000 are already basically set in the same universe: Excessively grimdark, with a corrupting evil force controlled by four remarkably similar demon gods. Conrad=Nurgle, Slan=Slaanesh (duh), Void=Tzeentch, and...okay, Ubik is basically tiny Tzeentch, so we're lacking a Khorne, but Zodd is already a Bloodthirster. Guts and Griffith are the two missing primarchs.
I've put way too much thought into that. Basically the only thing that would make Berserk more badass is if everyone had bolters.