I could suggest Tersculpt... old editor I know but it is detail based, so you won't have the issue of being stuck with a colour map and detail map, it's 6 detail textures IIRC.
http://forum.thegamecreators.com/?m=forum_view&t=122906&b=5
A while after writing Tersculpt though, I came to a stark realisation... I bloody hate editing terrains... it's time consuming and tedious and it can be very difficult to be organic. I much prefer to generate terrains now, but I use pretty much the same shader as Tersculpt.
Consider that instead of sculpting a terrain to match your story, maybe the story should be sculpted to match your terrain... like do you really want to know your landscape inside and out?, isn't it better to make it interesting to explore for yourself while you develop?
Also - just storing a random seed number means you don't have to store the terrain, a 1024x1024 terrain takes up about 4mb just for the height map. Maybe you could make an object placement editor for a generated terrain, but have it apply trees and foliage etc automatically. Seriously it could take hours to make even a small terrain, a 1024x1024 terrain is a mountainous task - it could be a project killer, whereas generating terrain removes that burden and makes developing it much more interesting I think. Put this way, I've moved my project onto DBPro from Cryengine, because I can't face creating those huge terrains or even using the built in generator - I'd rather mess around with my own generator, texturing system, and try to get things as organic as possible, and for me it's just fascinating, especially when you find a particularly nice cliff face or shoreline.
