He's the only surviving Lannister and still technically married to the head of House Stark, and there's always been the theory the Mad King raped his mother (hence Tywin's hatred of him), which would make him the most suitable Targaryen (let's assume Dany needs to be put down, and Jon Snow aka Aegon Targaryen says "I doon't woent it". That neatly resolves forever the dispute among Targaryens, Lannisters and Starks (or Plantagenets, Lancasters and Yorks). He's Henry Tudor.