Although both fighters are past their primes, this is still a compelling and would probably be a very entertaining fight. This fight really needed to happen 3-5 years ago when both were still relevant contenders. Would've LOVED to have seen this fight then. Still I think both guys got enough left to make it an action packed fight, and Rampage has got the revenge factor.
I say Rampage KOs him in the second or third round after a good back and forth exchange.
Shogun seems to be a shadow of his former self. Injuries, mileage, and horrendous damage taken in fights have slowed him down considerably although he's still dangerous. He's been incredibly hot and cold during his entire UFC run, looking anywhere from ferociously competitive to dominant in one fight to lackluster and worn out in the next. You never know which Shogun will show up but all points seem to indicate that he's deteriorating as a fighter. That being said, as was already mentioned, Shogun is ALWAYS dangerous, and has the skillset to either TKO page or if he fights technically, beat him in a decision.
Rampage would be the favorite in this IMO, in spite of the horrendous one-side loss to Shogun 10 years ago. Rampage has improved significantly since then and while he's not exactly his old self either, seems to have declined less overall than Shogun. Though Rampage has looked a little lackluster in recent fights, he was hardly blown out by anyone except Jon Jones and is also always one punch away from ending a fight. I think if this was to be his return to UFC fight he'd use that as well as the revenge factor as motivation and come in ready to destroy. This would be a good fight for him and probably give him a nice boost back into the UFC.
So overall I say Rampage, but Shogun is still dangerous and very talented. Can never really count him out against anyone. This would be a good fight even in 2015. UFC should make it happen.