Natami style SuperAGA card for A4000/A1200?

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If you can have someone make an Zorro-III card with an (Xilinx) FPGA with an suitable video output, an RTG replacement should be doable.

Now if one put the CPU, and RAM onto the same card there's less inner workings to deal with. But then it's essentially a new computer. So you could essentially just buy an FPGA Replay.
I think it all depends on how much of the original hardware you want to keep using, chassi, keyboard, mouse, Zorro-slots, CPU, etc..

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I suspect at the point that you succeded with an upgrade to A4000 to give it hotter graphics. It's not much left of the original electronics except the keyboard and power supply

So work the other way, make an Zorro-III expansion board for the FPGA Arcade. Then you will have any graphics (and CPU) that can be coded for.

Faster chip-ram might be an idea, but it also requires that the graphics and CPU chips makes use of it. One approach could be to use RAM fast enough to complete an access cycle fast enough to simulate dual-port RAM for CPU and graphics simultainously.

As for AAA:
https://en.wikipedia.org/wiki/Amiga_Advanced_Architecture_chipset

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Back to the starting point again..

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What's a Graffiti card? especially how does it work?

As for rewritting.. no source - dead end. And a lot of work.
The only realistic path is AROS.