There have been a few cool crossovers in the past. I first remember Dark Horse doing it with Batman and Predator, AvP etc. Then I can't remember who did Robocop v Terminator and Batman v Punisher. It's all cool if you ask me.
So, be it an alliance or an offensive, which characters from different labels would you like to see put together, what would be the situation between them and where would you base it? And... whyyyyyyyyyyyyyyyyyyy???
They can be a duo or an opposition. Personally I'd love to see Punisher v Predator:
Punisher hunting down a druglord in New York on an unbearably hot summer day. Starts a block war, wiping lots of street soldiers out and making his way up to the top of a huge apartment block. Predator gives it some. Wipes out the druglord before Frank gets a chance. In a sudden face-off, Frank kills the Predator and is freaked out at the sight of this ugly pussyface. Later on, enter Nick Fury, covering up the situation and hiding the evidence, knowing already who the Predators are. Punisher is so screwed and he may not know it, so Fury pays him a visit and explains the situation.
Frank probably has a huge alien bounty on his head. And they're right. Fury suggests a plan to draw out the alien invaders but Punisher tells him. 'nah, I'm busy street sweeping... you know, that never ending, crappy paying job I've been doing all my life?!' and arms himself up to go to work. But an encounter with another Predator ends with him nearly dead and rescued by SHIELD. Fury saying, 'you're welcome, grumpy pants,' offers Frank his plan to draw out the alien threat under "safer circumstances" so that Frank can go head to head on the monster's own terms. Frank agrees, liking the sound of having SHIELD tech to play with, not to mention the predators' stealth cloak and big scary bastards to kill legally.
Just what he didn't expect... this deathmatch arena will take place in the Florida Everglades in the dead of summer with only sparing communication with SHIELD's satellite system to help out. Frank goes guerilla style, what he's best at. But before long, he's up against more than one invader. But for some reason, I can see Frank doing a runner, having the Preds chase him to a location full of druglords and their soldiers and using the Preds to blow the tits offa them and save Frank the job while he starts popping off the alien hunters.
Note: shut your face, if Danny Glover can beat one Predator hand to hand then Frank can take several :p *blowing raspberry sound*