Problem: Maps can belong in multiple categories, as well, we would generally want maps to be as easy to install as possible.
Megaglest has a filter that lets you filter maps by the number of players (so when looking for a 8 player match, you can set the filter to 8 and only maps that support 8 (or hypothetically more) would be shown).
Biggest problem is that (1) things must remain compatable with current folder structure, for both ease of use and so installers, etc work fine, and (2) it would have to bear in mind that many maps would belong to multiple categories... Plus, there's the final flaw that there is so little room on the custom game screen, and map categories would just add another dialogue box... Though for the most part, I do see how categories would be VERY useful, particularily since Military now has water units, but only maps that were specifically made for water units work well with them, and, while all of them also have a scenario that shows the map, its still harder than a category.
@Chupa, place the scenario maps in the scenarios directory. That way it doesn't appear in the map list, and is only playable in the scenarios (new feature to 0.3.1).
