So, what I learned so far from your reactions:
1. V9978 wouldn't have been as powerful as the V9990, because of all the trouble of keeping it backwards compatible. Perhaps it was even the reason why it got delayed/canceled?
2. Ideally, one could imagine the V9978 now as simply a V9958 and a V9990 in one chip, with some register to select which layer (V9958's or V9990's) is visible (perhaps also a register to tell which layer is up front).
Of course we could only guess what the specs/details would have been of the original V9978... or is there still someone around in contact with the Yamaha or ASCII people who were involved? 
Superimpose requires that both VDP's are synchronized. That's near impossible with totally different chip, that each have their own screen resolutions, sync rates etc. Like a VGA compatible mode on the V9990, how would you 'align' that with a PAL or NTSC mode from V9958? Forget it.
I could imagine some 'extended' V9958. But what's the point? If a V9958 is what you need, then using an uber-V9958 would shrink your target audience to <1%. If a V9990 is what you need, you want the real thing & all of it. Only exception is extra's that aren't necessary for things to work. Like remove # sprites / scanline limit to reduce sprite flicker, speed up blitter etc. Trying to combine the two is like trying to roll an ARM and x86 cpu into one. 
Impossible, or the franken-result woudn't be compatible with either of them.
So switchable VDP makes most sense. Perhaps share VRAM or video output. Or side-by-side so you can do a dual screen setup.
Of course we could only guess what the specs/details would have been of the original V9978...
Most likely "V9978" was a working title of sorts. And then while drawing up specs, @ some point compatibility with V9958 was dropped (see previous comments), and name changed to V9990 to avoid the suggestion of 9938->9958->9978 compatibility. Imagining a V9978 as an actual chip that was on the design table once, is giving that title "V9978" more weight than it deserves imho. Whatever it was or wasn't, on paper or in the heads of designers, it never made it to silicon. End of story. And the V9990 which did, is a totally different beast.
Btw Wikipedia says Yamaha re-targeted the V9990 with some success towards VGA card market. Anyone ever seen a VGA card with this chip? 
The first problem is they conflict with the ports 64h and 65h of Panasonic MSX2+ with the turbo mode.
Interesting, what do you expect will happen if one try to use V9990 with panasonic MSX2+ ?
In fact this is the ports 40h and 41h that are used by Panasonic MSX2+ for the turbo mode. So no conflict.
