Dark Claw(Wolverine/Batman)
Well the Batman/Wolverine was already done in the Marvel/DC Amalgam crossover a few years back, heres a pic, the were called Dark Claw but me personally would like to see a union of Green Lantern(ring) with either Reed Richards or Dr.Doom, since the ring is only as powerful as the imagination of the wielder, how more powerful can it get with either one of the smartest men in the Marvel Universe!
Hawkman and Bloodstone. Bloodstone was immortal, so take that aspect, but set him in Egypt, and give him Hawkman's crap, and the bloodstone, of course.
Born in Egypt however many thousands of years ago, the man that would become known as BloodHawk is an immortal possessing the bloodstone and thanagarian weaponry, (i think it was, can't remember all of their respective backgrounds). And blah blah, lives in the modern day fighting evil and blah.
They later put a slant on the bloodstone and many other of Marvel's gem powered heroes (Moonstone and Blue Diamond among others) that these stones were worn by galactic guardians and some weird stuff, so they could merge some of that with Thanagar, or whomever it was that Hawkman originally encountered in Egypt, starting his story.