Here's what I know:
As you are aware, the editor as it stands has no provisios for certain type of modifications, like for example, you can't set up diplomatic options, see the whole map, set city production levels, etc. Biggest of all, there's no event editor. All these have pretty much discouraged the making of scenarios in Civ3.
There are RUMOURS of a supposed 2nd expansion pack which will feature a much more robust editor and pre-made scenarios (the japanese medieval and ww2 secnarios).
Warp Storm (if he shows up) knows much more about this. There are also many threads over there at CDG
www.cdgroup.org