Hi Jakub,
First of all, thanks again for all the work you've done so far on the core.
As for what improvements I'd like to see, there are only a few:
1. "Hard Drive" support via the SD card.
2. Sub directory access in order to group certain ADF files together (eg, Tower Assualt files plus save game disk)
3. 2nd Floppy support (if no hard drive support) in order to make playing games like Dune or Colonization possible without contracting the Minimig equivalent of "disk swapper's wrist".
4. Core file selection on bootup (to allow access to several cores such as PAL, NTSC or a Arcade core).
5. Solve the bug that breaks Menace (well, it is my favourite shoot-em-up). :-D
Cheers.