What is it with people and ... USB".
IMO at some moment people will get interested in adding compatibility with CDTV, CD-i, Sega CD, NeoGeo CD, Turbo CD, etc. so an interface to plug a CD-ROM would be needed (it is feasible to use the memory card, but adding the capability of using real CDs will add legitimacy to the hardware). SCSI is legacy technology, SATA is so new there are no cheap CD-ROM drives for it. That leaves IDE or USB and in my opinion the later is harder to implement but can be used for other peripheals later on so it would be the best choice.
You will need around 200Mhz for USB
No you don't, I remember someone hacked an USB port into his (unaccelerated) A1200 and it worked albeit it did not reached the maximum USB 1.1 speeds, but IDE would be just as slow.
Then you want IDE
It would be redundant with the SD and USB interfaes.
128MB of ram
Would be nice :-D
AGA
Would be nice for some workbench applications, albeit I do not think it could run 32 bits apps/games/CD32.
why not just Design a totally new board
I name thy minimig 2.0 :-D
of course you only want to pay US$30.
I think that if a price range between 50 and 99 USD for a machine without a floppy drive or CD-ROM drive or keyboard or mouse but with a simple case, a 5 volts DC adapter, a sega genesis joystick and a SD card with some games, an AROS kickstart and OS would sell like the C64 Direct-to-TV. Maybe I am dreaming, I know :-)