Sd card with roms. And a portable feature.
I would pay 500 usd for such a thing.
And I would DEFINITELY not. Although a portable with a screen, controller ports, and video out could be interesting.
First of all Retron 5 isn't FPGA, it's just software emulators. I can already emulate all of those systems on my own computer (ya know that device I'd be using to put ROMs on an SD card anyway).
The FPGA piece is interesting because it has the potential to be even more accurate than software, but someone has to make the core and spend time tuning it. Apparently there is a good XE/XL core out there. I've used a 2600 core that was awful, and I've never heard of anyone working on a Jag core. Maybe Kevtris has one.
The point is that these devices have to be really good to get the old timers involved, and I think most of the casual fans in North America have a Flashback by now. The demand would be VERY low anywhere else (unlike the NES).
Another difference from the NES is that they fail at a much higher rate (cart slot, blinking light), so replacement hardware is more desired at this stage.