Now I know this is a ways off from being useful for Black hw. But Pistorm has recently started shifting towards the emu68 bare metal JIT emulator.
https://www.youtube.com/watch?v=aVqUihLZa3sSo far they're getting 800mips. Now the downsides pastor is currently only for the original DIP 68000's. and emu68 when enabling MMU support cuts its speed in half or more. 800mips down to 400mips seems fine to me coming from a 25mips Turbostation. Also all the peripherals would still run at the same speed. Particularly the SCSI, video memory and ethernet.
I wish I knew hardware, because I would love to drop in a equivalent of 100Mhz+ chip in place of the 68040.
So for now it's something to keep an eye on. If they start to work on an '040 drop in, we should all pitch in and make sure it can handle the pinouts needed for NeXTs.
I also think this looks like a good approach.
In my sleepless hours I'm relearning KiCad by adapting the 68000 socket to an 040 PGA. There is GAL code that can possibly be reused or adapted.
I have been looking at 040 50mhz chips and the rampant fakes in general, and this seems easier to than a Nitro clone. Not to mention the other benefits like network booting from the Pi, NFS and such.
Update: Even if I were triumphant, there's a huge gap in software to close before this will work as a drop-in replacement, and I'm not clever enough to do it.
A Nitro clone will be significantly simpler than this project. As a starting point, z3660 is probably better suited than Pistorm. It is based on the A3640 / A3660 accelerator, which comes with 030->040 glue logic, and adds the necessary logic to add a zynq module onto that 040 bus.
In which case we need to gift shanshe a next machine to convince him to create a NeXT3660 for us :)