Sorry guys but it seems this has been discussed like a zillion times.
There's a no-go with an implemented core using the actual FPGAs that could outperform a plain 020 with AGA.
There's a no-go with real native hardware fully compatible with AGA, see the S-AGA on the Natami.
At the end nothing like a new Amiga portable hardware will be made.
FPGA's can't achieve (for now) what we want at an affordable price.
Design new native compatible chips and bake them is stratospherically expensive.
However, i'm not sure if someone came with this idea so far.
What about using a low-cost PPC as CPU, and have the video and audio implemented
in two different FPGAs?
PPC as a CPU will grant a limited 68K backwards compatibility, plus it would be able to run MOS or OS4.
The PPC CPU could even be set to run in two different modes, native PPC, in case OS4 or MOS decide to add support, or an emulated 68K core.
As for the implemented audio and video, as long as they would be implemented in two different chips, the overall performance would improve quite a lot.
As long as they would be implementations, extra features or extra operation modes controlled by software would be possible.
However, as long as i'm not an expert, i don't know the drawbacks of this idea.