Because of the way map data is stored, I've found that the trick to adding detail in one place is to remove detail in another. There are some little peninsulas that the player would never walk on that can be oceaned over, which could free up a fair amount of space if done right. Likewise with snow-capped mountains, etc. An entire row of ocean can be written with only two bytes; but a row of ocean, then west bank, then piece of land, then east bank, then more ocean takes seven bytes. That kind of thing adds up.
But also, that's why yousei's editor would just automatically expand the ROM. If you're not constantly paying attention to space, you can go over pretty easily.
Pinkpuff put a pretty ingenious device into the location map editor: a "space remaining" window. In my project I've never even come close to filling location map space up (I think I have around 5700 free bytes, it says), but I know others have had problems getting enough map space for their projects. That would certainly be helpful in a world map editor, though, if it were possible.
We have EVEN MORE themes now, but some old ones are gone...