There's actually plenty of map space - for locations, anyway - as long as you build intelligently. One great thing FF4kster does is automatically consolidate data banks like maps after you make edits. It even tells you how many free bytes of map space you have left.
The only aspects I've found that would benefit from expansion would be text and monster graphics really (as in, adding new images to the existing list). I mean, expansion might come in handy for other things, too (like, say, having more than 255 events), but doing so would require ROM expansion in addition to significant assembly manipulation.
