Starcraft – Change tileset for already-created Starcraft 2 map

The custom game selection will allow you to draw from maps that you have uploaded to battlenet via the Galaxy Editor. As I understand it this is exactly what you want to do and it sounds like you do understand how to publish a map.

I believe there is an option to publish the map privately for custom game use, though I haven't check since Beta.

Balance is most important. Don't give someone a better start location than others.

This could include better choke points, better expansions, closer to rich mineral nodes.

Besides that, generally, modern maps have a main base, and a slightly less defendable expansion, but that's not necessarily required.