I am one of the people who did like the Ninja gaiden suit x-over on Halo 3. I also think that the glimpse we get of Mster Chief's face at the end of Halo 4 strangely resembles Big Boss's recent renders of his face; which imo begs the question, would a x-over with Metal gear be so far-fetched? And of course, the most common one would be, will we ever get to see a star wars clone trooper armor set?
Personally, I hope it doesn't go any farther than Hayabusa in Halo 3. Although I'd like to see it return, I thin any other crossovers would harm the immersion of the games and the feeling of being in the Halo universe.