No limitations! Can belong to any company and any platform.

Personally, I think a Halo prequel where you find out that the forunners were created by Bowser would be amazing.

Pokemon  X Call of Duty. I want to  shoot Gary Motherfucking Oak in the face. That bastard.

"The forunners were created by Bowser" - Ha ha!! That would be hilarious!! And would finally get me to buy another halo!

Metroid X Halo

God of War X Yoshi's Island 

Metal Gear Solid X Kirby

My dream crossover came true already: Phoenix Wright x Professor Layton.

Now, I wouldn't mind seeing Monster Hunter x Pokemon.

Castlevania x Metroid

Finally get a real Metroidvania.

I just wanna play a Nintendo vs. Capcom game.
There could be NvC in the style of smash bros. and CvN in the style of the Marvel vs. Capcom games.
It would be cool to see the shotos vs. the plumbers haha (Waluigi vs. Dan!!!)

Attack on Titan X Pokemon
It can have that boss hunt feel to it as the characters from AoT go slaughter giant Pokemon