Here's my personal wishlist: emulate the FDC chip of the subject line and make the disk interface of the ZX Evo switchable between Beta and +3 compatibility.
The ZX Evo provides a good compatibility with the 128K, it can be made even better by adding the possibility to load the DSK format either from disk or from ramdisk.
It may feel like a step backwards in some way, since the Betadisk interface gives much better storage capacity, 640K vs about 180K in the +3.
But considering the number of *.dsk.zip and *.trd.zip files on WoS, it would double the amount of games playable from real disks or disk files on the SD card.
Although the original floppy drive in the +3 was 3" with an Amstrad CPC/CPW compatible disk layout, regular 3.5" PC floppy drives were also usable as an external B: drive.
FWIW, there exists a hardware that adds such an FDC interface for the +2A. Links:
http://www.worldofspectrum.org/BackToThePlus3/
http://www.simandl.cz/index_a.htm
http://www.datasheetarchive.com/dl/Scans-007/Scans-00158373.pdf
I am not familiar with FPGA programming. The question to those who are: how much work would it be?
In English, they say "vote with your wallet". I would be able to sponsor some of the work time needed for this.