A little update for those interested...
we are currently prototyping a true 3d engine using
irrlicht.
Here's a quick hack of a walkaround map with collision detection and a fixed camera angle:

Most of the game will probably be like this. (Not art-wise, the map and the character are just some stuff from quake.)
Here's the "world map" we've been prototyping, which will allow you to travel between 'floating' continents in an airship:

(Muller and I made the crappy looking models, the cool jet is from vegastrike.)
The plan is to finish these prototypes as proof-of-concepts in order to get a better idea of how to pull this off. If we find that we can do everything we need to do with irrlicht, all 2d plans will be scrapped for good.