FFH isn't going to change, and its where everyone who plays FFH starts. This would make it an excelent base for Rise of Darkness. FF has an excelent DLL that would allow you to do alot of cool, and neat things. It is also advancing slow enough now that modmods can be based off of it, and they won't have to be constantly updated simply to keep up with the base mod. FF+ though seems to have simply too many things that keep being added, which may make it hard for people to adjust to Rise of Darkness. I wouldn't recommend orbis, simply because you would have to radically re-design your mod to make it fit in, and LENA for the same reason. Wild Mana I wouldn't recommend, simply because I have no idea where Sephi plans to go with it - it does have a great AI, and the mana guardians stuff is cool, but without knowing what his further plans for it are, I personally wouldn't try to base a modmod off of it.
So I guess my vote would be for FF or FFH - your preference, the one that isn't going to change much, or the one that has the best DLL (toolwise), but will change from time to time.
-Colin