I've tried and failed to figure out what the heck I'm doing in GtK Radiant, so I think I'll just post the map ideas I had here. Maybe someone will like one enough to make it? I figure this could be a good place for map makers to go for inspiration.
My first idea was a 'funhouse'. Not necessarily a thematic Western locale, but it would be... well, just fun. Signs on the walls would give suggested strategies. For instance, there would be a T-shaped hallway, with one 'leg' having a dead end. At the end, there is a picture of a gatling gun on the wall (an engraved wood panel would be preferable). So, the strategy would be to buy a gat and set up on that hallway junction. Your back is against the wall, so you could just worry about shooting others on that hall. However, one floor above, there is a hole in the ground directly above the gat. That hole is not big enough for a person, but it is labeled with a stick of dynamite and a downward-pointing arrow. I'm sure you can see how that would be used.
My second idea was Train Junction. A wide expanse of flat land with several sets of tracks running parallel and a medium-sized train depot building near the middle. Plenty of cargo cover around, too. The trick to this map would be that every once in a while a train barrels through on one of the track sets, killing any lawman or bandit unfortunate enough to be standing there. Not only would players be fighting each other, but they'd have to be aware of their environment. I admit I'm not too sure of the feasibility of this idea given the tools you have, because as I said I wasn't able to learn them very well. Perhaps if it's possible, a train could stop at the depot every once in a while-- loaded with money. Players dash to take the train and the wealth inside, but if you get caught in the train as it moves off again, you're a dead man...
Anyone else with ideas could post them here too. Although, I'm sure those who can map are very busy already.