Suggestions

[Ton]

I generally try to keep sections sorted like this:

Top: Playable Character(s)

If it's a fighting game or racing game I put them under "fighters" or "racers" even if they're not playable. This could also be "Units" in the case if tactical games.

Next: Non-Playable Characters

In an RPG this is self explanatory. In a side scroller this might be Peach or Protoman. If there is only one npc sheet, this may be relegated the the Miscellaneous section, as one sheet may not warrant a whole section.

Enemies etc.

This could also be split into Minibosses and Bosses if there are enough sheets to warrant it. Some games, like the Megaman series, have a Robot Masters/Mavericks section, because they're a special subset of characters.

Stages/Backgrounds

This is tricky. Stages generally includes both back and foregrounds. If there are layers, there could be separate back and foregrounds sections. This could also be called "Stages" or "Levels" based on the game and how it's presented. Map sprites may be here, or in their own section if there are enough of them.

Special effects

Again, if there are only a few of these they could be in Misc. Could be magic, spells, etc.

Miscellaneous

Things that don't for anywhere, or just categories that don't necessarily warrant a whole section (see the note in NPCs). Could be items, cutscenes, menus, introductions, endings, beta sprites, etc. If any of these categories are large enough, they could be spun off into their own sections. (Beta sprites, Cutscenes, etc.)

Tiles

This could go after backgrounds if they're background tiles. I generally out them at the very end because of the technical aspect involved. Casual users aren't going to care about this, especially if they're zipped.