I don't know if it's been said already, but RPM tried to go 3D with RPGMaker 3 on the PS2. Now, granted, technology was nothing compared to how it is now, but I just don't think the 3D RPGMaker market is big enough. If you ask me, pixel graphics allow for a lot of customisation, a feature that anyone can take part in with relatively easy to use software. Whereas customising 3D models is a little less mass-market friendly, and I wouldn't see as many people doing it if the next RPGMaker were to go full-3D.
If you ask me, next installments should focus on adding more customisation power to the creators, letting us do more and more with software and aesthetics we're already familiar and adept with, rather than trying to dive into relatively unfamiliar and financially risky territories.