I'd have it set on a mining planet, post Alien³, with a diverse bunch of humans, a pair of Predators and a trio of Aliens. The operations been going a for a good few years, put together by a mysterious smaller company that pops up during WY's impending Wal-Mart doom. One of the monthly cargo pods contains, you guessed it, three eggies. In the scrub outside the colony there's an on-going chase between two Preddies. A bad-blood (think Predator serial killer) and his (less dickish) counterpart that gets even more interesting when the come upon hell. Awesome ensues.
I like the idea of having a Predator that doesn't use all the fancy tech. No plasma cannon, no cloak, no mines; just a mask and a blade, running around killing anything and everything. The situation forcing the Predators to work together to take out the Aliens isn't a shabby idea either. As for the Aliens they need to get back to being, well, alien. Plus that unnerving tension when you have to outsmart the beasties rather than just blow them away.
Oh and this is a reboot. I choose to throw what came before in the cornfield and move on.
