All cartridge based systems with a significant fan-base have been covered, the 2600, 5200, Colecovision, and I think the Intellivision (Hell, even the Vectrex has one)have all been covered. The 7800 and Jaguar don't have anything, but their user-base is tiny so potential sales are also tiny. I mean realistically how many of those consoles are hooked up, functional, and have owners that want to play them? There weren't that many sold to begin with.
Most of the popular 8-16 bit pre IBM/Wintel dominance home computers have solutions as far as I know.
Handhelds, I'm pretty sure the Virtualboy, Wonderswan, and NeoGeo Pocket have solutions, but I don't think the Lynx does...but while it was an interesting handheld from a historic perspective (and equal-rights for left handedness) it was not a hit by any means. I can't imagine that many units still working without an LED screen mod.
Realistically I'd say if Krikzz is interested in making another flash cartridge, the GBA would be the best bet. It has a huge library, and many units are out there, and still functional. A large amount of hardware is capable of playing GBA games (3 models of GBA, 2 models of DS, GB players on GCN, then there was that portable DVD player...) So, yeah big base, big sales.