My problem with Naked Rowlf, is that even though I would love a big Rowlf, it would seem weird to me to have a plastic Rowlf that size. For some reason I can stomach plastic rowlf in 6-8" regular figure or 3" PVC, but anything larger and he needs to have fur! He's a dog, he needs to be petted! And since these Megas are being based on the basic sculpt of the regular size figures, I doubt he's gonna be fuzzy. Plus since he's so heavy-bottomed, he'd really be huge in Mega form. So not saying I don't want him, just saying that I don't think he'll be coming in the form I'd prefer him in.
I'm not gonna play detective for the 4th Mega, I'll just speculate/wish instead. I'm thinking Janice would be a great 4th Mega. Since she's so thin I'm guessing they'll have to sacrifice some articulation and possibly modify some of her design in order to keep her from having the "Kermit Syndrome." A larger size would allow them to make her more true to form with better articulation. So at this point my money is on Janice. Plus with the exception of Bunson, she fits pattern of releasing a Mega version of a playset figure. Plus she's female and could possibly appeal to a different audience than the previous set of Megas.