I could see Jaime being the one to deliver Gregor's skull to Dorne, that's the only thing I can think of that would make sense.
He's got a lot of content left from the books though. Cersei begging him to be hand, him suggesting Tommen/Marg get married, his drama between Cersei / Kevan / Loras. There will be a lot of travel scenes as he's supposed to stop by Harrenhal, and then Lancel admits that he was banging Cersei before they arrive at Riverrun, then there's all the shit with Edmure / Brynden. We can end Jaime's S5 run by having him get a letter from Cersei which he burns, making us all fist-pump. S6 can have him handling the shit at Raventree between Blackwood and Bracken, then a long travel back to Riverrun, where he meets Brienne and... well, hopefully he doesn't get killed by LSH :/