Since they are making a whole new product to port VX Ace games over to mobile platforms, which itself is probably a few months away from prime time, I really doubt we'll see a new engine until at the very soonest, the end of 2015.
They wouldn't put a ton of cash and time in development and testing, just to have to at the very least do it partially again for a different RPG Maker game engine. Enterbrain probably isn't a large enough company for them justify such a re-investment.
After all, at the very least, it will probably change the RGSS interpreter to V4 which will probably have API differences which must be incorporated into the porting engine. Not including any Tileset changes, of which there were significant ones from XP to VX Ace (Autotiles).