I know at some time there was talk of putting a fast G3 or G4 into one of these accelerator boards - instead of just the PPC 603 or what. Did anything ever come of those efforts? With all the faster PPC (1Ghz + ) chips around now, would it be possible to make such a thing? It would really bring an A1200 along.
Sure it would be possible. Elbox's "Shark PPC" would have added this to the Mediator using a re-badged Sonnet Cresendo 7200 PCI with G3 or G4. But you can no longer purchase those (I tried, from their webstore, but when you try to "check out" your shopping card, you get an error). I even called Sonnet and talked to a sales guy about availability, since the website said they were available, but then at checkout they aren't.
He never ended up getting back to me.
It's possible to put a faster PPC chip on the Blizzard PPC or Cyberstorm PPC (some have done it, and I have such a 366Mhz cyberstorm PPC board). But it's difficult to get them working reliably - you have to get very good RAM. It's hard to find reliable fast SIMMS nowadays. Most of the sticks I have tried (over 20 32MB 72-pin simms) lock up after awhile. The memory interface on the cyberstorm PPC is lame. It's fixed at 60/70 ns and can't really take the higher speeds very well without VERY GOOD ram and proper/extra cooling.
A new amiga-specific development would have to be done to create a new PPC accelerator for the A1200/A4000. To date no one has been willing to take on this project. Probably because the market for such an upgrade is small (probably less than 500 units) and the cost of developing such a board would be very high. It would be very doubtful the investment would be re-couped.
Even some people had contacted DCE a few years ago about re-starting Cyberstorm PPC/Blizzard PPC production, but some of the parts are no longer available and it's not ROHS compliant, so it would need to be re-designed. Not to mention DCE washed their hands of all Amiga stuff (there was a rumour they threw all their Amiga stuff in the garbage....).
Even if you had such a board you would need programmers to write support for it.
I'm the first person who would want such a board but unless we are able to come up with probably a couple hundred thousand dollars it's not going to happen

There are a couple of companies that could possibly develop this, though.
AmigaKit could sponsor development. E3B or Individual computers could potentially design it. But it would require significant investment.
AmigaKit has come through with the ZorRAM board, which has been nice. I hope their sales are OK so perhaps we can have future developments sponsored by them in the future.
E3B has released their excellent Deneb hardware. Not sure what else they have planned. By the way, they also designed the ZorRAM board in conjunction with Individual Computers for AmigaKit.