An indoor themed 'thief den' would be a grand addition. Adds a good indoor set to the mix and would mesh well with the others. If room parts are put in (corners, midwalls, floor) we could make isometric interior houses as well which would be neat. Another neat thing for the caves set is rail-road and rail cart props. That way the caves could be a dark mysterious abandoned mine, animal den, magma cave, mushroom haven, hideout, and crystal cave all in one with the existing props you have. Caves are pretty versatile, you could probably sell isometric prop packs allowing more flavor for every map :)