Pete Foley worked on something similar while at Apple. See "Other Projects" at

https://web.archive.org/web/20190102164746/http://www.byrdsi...

and the picture "Turbo 6502" in the slide show at the bottom of that page.

I mean, even the IIGS wasn't really an Apple II in a real sense. Classic Apple II programs did use the IIGS' CPU, but were otherwise basically running on the MEGA-II (which I think is also how the Mac LC did Apple compatibility, although I've forgotten). Writing programs for GSOS had more in common with Mac programming than the Apple II, and properly using all of the GS's advanced sound and graphics required entirely different code from an Apple IIe. The 65816 also had a lot of practical issues stemming from the global nature of the 8/16-bit toggle that would've only gotten compounded if you tried to naively extend the architecture to 32-bits.

In the end, yes, I'm very curious what extending the Apple II line would've looked like, in an academic sense. But I suspect that, by the mid-90s, the "Apple II" would've been as technically unrecognizable from the Apple IIe as late 90s PCs were from the original XT: lots of echoes and lots of backwards compatibility, but extremely different when properly and modernly used.