Anything vs Anything is bound to sell, crossovers are huge, and everybody wants them and likes them. Alien vs Predator, Freddy vs Jason, and in video games they're even more popular, EVERYONE wants a Samus Aran vs Master Chief, of course they could just watch Mounty Oum's "Haloid".
If you want to make something that'll sell, but since all the good ideas are taken, then make a crossover and you WILL get some money.