One alien, based on the Big Chap design.
One predator, based on the original design.
Predator should have no projectile weapons, or at least have them disabled at the beginning of the film. Wrist blades, knives, and thrusting/dueling spears only.
Humans that are not heavily armed but are competent and memorable.
Gory but done in a manner that evokes horror and is not cheap like what we saw in Requiem.
No human-predator team-ups.
Horror feel of the first Alien film mixed with the filthy, animalistic feel of the first Predator. Maybe it could be set on some weird, Beksinski-esque lifeless world that oozes hopelessness:
This film should just be called Alien/Predator, no 'vs.'