If you want to do 3D gaming, GE wouldn't need just a "rewrite" or an edit done to its source code. completely new software would have to be developed. There would have to be a method of creating 3D models INSIDE the GE, painting them, designing them, rigging them, and animating them. Which is all MUCH easier to do in 2D!
I've worked with Blender 3D for a couple years and trust me, it's AMAZING stuff! But it took me FOREVER to get where I am, and I'd still consider myself a beginner. And yet with GE, it took me less than a week for me to start getting the hang of it (to a degree). Personally, I don't think GE should even try to develop on 3D. Leave that to the people who make games! Make your animations and your backdrops look great. Bee-Ant's Darkboy Agent Training Demo is a GREAT example of this. It's code and imaging. It takes a lot of talent, and for lack of a better word, it's a visual "trick".
2D games can be complex and incredible in their own way. Instead of taking all that time working on the most visually entertaining 3D models you can think of (in the 3D world, EVERYTHING NEEDS TO BE VISUALLY APPEALING AND FILLED WITH DETAIL!!!), you can concentrate more on the quality of the game. Games like Rune Factory are actually 2D, but created at such an angle that they APPEAR 3D. They have amazing gameplay, but they aren't 3D.
If you want 3D, try Blender out. It takes forever to learn, but it's open source.
http://www.blender.org As for 3D in the GE? No, that would practically be a waste of time, in my opinion. Many tutorials would need to be made for using the 3D engine, and so many childish games for 3D would be released that it would be unbearable because once everyone finished with the modelling, sculpting, and animating, they'd feel so worn and tired of looking at the models that they'd hardly care for the quality of the game. Making decent 3D games requires a *lot* of experienced people. to make it work. GE should continue to develop in other ways, such as consoles it can export to, perhaps more common variables, and plenty of other ideas users have come up with. But not 3D - if you want 3D, be creative and do it yourself in GE by visually "tricking" those who play your game, or just go and learn Blender.