Considering how the Evil Dead series is ALREADY tied into the mythos of the F13 franchise, I don't see how a mash-up is that hard to come up with.
Now I hate as much as anyone to bring up the godawful Jason Goes To Hell, but that film does contain the Necronomicon from the ED series and I believe the dagger from ED2 as well (can't remember, haven't watched JGTH in forever). The groundwork, for better or worse, was already laid a decade ago.
Besides, if they had actually made this into Freddy vs. Jason II, I'd have been glad. I'd much rather see Bruce Campbell as the hero of the film than a bunch of no-talent C-grade (or worse) teen actors that will never amount to anything. Who wins in FvsJ? The cliched teen characters, naturally. Why? Because they wouldn't want to alienate ANOES or F13 fans by having one or the other triumph. So if we are ever going to get a sequel, I hope they at least have a hero or heroes worth rooting for......for once.