In no particular order of interest:
1. Jag CD support - especially given this does not work through the Jag CD slot
2. Support for use THROUGH Jag CD port. I know i asked for this before in twitter and you said you can run an interrupt based poller or something like that as a fallback, that will load things slower, but will kill CD support. Fine with killing CD support, as long as you have the unit installed. I just do not feel like selling the CD add on because the cart loads CDs off the SD and also hate having to install and uninstall the add on seeing what kind of pain it is to install it 'right' (if you have one, you know what i mean). This way, i will be able to play roms through the cart and cds using the unit.
3. In game reset, ability to go back to the JaguarSD menu from a game. At the moment i have to power off/on.
4. Seeing as i am getting red boot screens (and you confirmed this is a bad dumped rom), perhaps some sort of 'rom test' to see that it is indeed a valid rom, if that is even possible?