Has anybody ever played sauerbraten? Thats the sort of community feeling (not exact implantation) that i would like to see (of course, i'm not talking about the map wreckers).
You basically got 8 or so people zooming around and making buildings. It was all a grid system (which outerra isn't really) but the feel when you built a huge city was great.
I don't honestly know how i would like the map-editor to be. I'd like to be able to import files (.blends or exports of such) into outerra, but then where are the files hosted if i'm playing multiplayer? Back to sauerbraten, you joined a game and the map was empty. Then you pleaded for 5 minutes for somebody to type "/sendmap" which would upload the map to the server. Then you would "/getmap" and download the map from the server. From then on all players would be in sync and free to save the map onto their computer.
If somehow only the changes were transferred, it would result in lower bandwidth.
On the notion of props, I firmly believe that there should be a large standard library (likely helped along with community contributions) that players can just pick and choose from. Sadly i don't think we're quite at the stage yet where each users computer is a server ( give it a few more years), so user-based hosting (not distributed, just client-server, with the server being the users computer (user being the person with custom content)) probably isn't an option. The integrated web-browser would be a nice catalogue maybe, with a service similar to google warehouse?
Placing wise we're all used to the standard found in RTS games. Click , hold to rotate, place. This unfortunately lacks detail and suitability to the ground. Another placing method would be using latittude and longitude, but unless there's an easy way to measure and record lengths and locations, this could get tiresome and it still doesnt' solve the problems in the RTS method (but it does make pre-built worlds easier, so maybe good enough for those who license the engine).
Customs props are necessary and a good idea. Obviously you're going to get some vulgarity and the like (*shudders*) but it would really make outerra (the game thingo) in my opinion. To more easily make 3d models of buildings destined to go in specific places, perhaps it would be an idea to be able to extract a low-detail heightmap that could be imported into your favourite 3d modelling program.
For the props database, perhaps it would be a good idea to make them all parametaric. If you're not used to some of the more advanced CAD programs this means that you can change almost all parameters of the object (such as the length/width, height, skirting board size/profile) and everything else will update itself and it will still work. For example, a minor and simple use of this would be to add length using an array, or change the profile of a doorknob (one of those round ones). The base 3D model information would be the same, so the file would only be hosted once, but bits of it would be altered to suit. This might be similar to
http://www.openscad.org/ or not, never used it, but it is similar to a carefully designed autodesk inventor object.
Wow, i've warbled on a lot.