Phoenix wrote:Revamp, maybe? The only other R I can think of is too big for boat (Rammstein). Revamp would be interesting since that may also mean Nightwish.
Nightwish is supposed to be taking the entirety of 2017 off. I wouldn't make that leap, especially since Floor has said she has no interest in doing a combined tour. (Of course, there was that whole thing with Nightwish being tagged in Kamelot's Instagram post...)
Running Wild makes the most sense. Nautically themed, most requested R on the wishlist, and relatively popular (unlike my preference, Raunchy). Riot and Refuge are also conceivable having done the cruise before. I could definitely see ReVamp, though, since I suspect Floor will devote some time to her own band with Nightwish idle. The only other ones that jump out at me are Rhapsody, who was on this year, and Rammstein, who as you said is too big.
EDIT: Actually, Revocation is a possibility. Haven't announced anything really heavy in a while.