Just as I was able to patch it to allow 30 levels, I could possibly make patches to support 8 keycards, and 8 items to collect. However, there will be more difficulties there than with the extra levels, due to hard-coded limits on the number of different tile types. But they're not insurmountable.
Making extra key cards would work really well with the holding onto keycards after exit patch... Then you could reserve those 4 extra keycards for special doors (probably on the map), and still have normal doors...