Just don't call it "engine". Call it studio. You want to have all the tools as well so it can be called "Original 3D Studio". But even programming those tools will be a tough job. I know it from programmers who did it for Ogre. It took them a lot of time (years) and they are still not mature enough the be distributed.
I could imagine that it will be easier to create it around Irrlicht than around Ogre but I don't have enough experience with it.
By the way: I don't like that some of you call this illegal. I don't believe that an editor for ogre or irrlicht is illegal. As an artist I highly appreciate such efforts because I would like to use such easy-to-handle editors for engines like Ogre and Irrlicht. Just don't call it an engine. It is a tool-set or a studio. It might be a wrapper but this is hard work as well if you do it right.
