I was writing this while Zozo...
If you mount a big FlashRom, you will have room for a lot of valuable Roms.
A must for you can be the
Danish Rom, a modification done on your own country on classic times. Probably you have heard about it.
We have already talked here about Epdos, that has a Rom version, but you also have all the classic programming languages, like Forth, Lisp and HiSoft Pascal among several assemblers like Asmon/Genmon, Heass and Fenas.
There are other Rom utilities, like ZozoTools(worth if you have a RTC installed, but with a lot other applications inside), File/Iview, HxC, all intended to make the use of the Enterprise more enjoyable, with several music players and graphic screen launchers. EDCW is a mouse controlled windows like environment that has some sound applications in it.
Of course you have some games on Rom, like Paszians(a solitaire cards game with mouse control...) and Kaszino, but also a way to put standard games on Rom.
But the most surprising Roms are SPEMU, SPEMU128, CPCEMU, able to emulate other machines with high compatibility.