So we've been building our levels with Tiled and the TMX Importer using sprite objects acting as tiles as Ashley & Rexrainbow suggested...but now that we have so many tilesets we're running into all kinds of problems - very long preview times, freezing when loading levels, poor performance, etc. So the only thing I can think of doing is replacing the sprite objects acting as tiles with tilemap objects (which didn't exist at the time we built most of the levels).
Well, unfortunately that's not possible with the tilemap object's limitations - namely how you can only use one tilemap per tilemap object.
Our own problem aside...Working with multiple tilemap objects on multiple layers is VERY cumbersome.
So, It'd be fantastic if we could store multiple tilemaps in one tilemap object (using a frame system or new menu in the property bar), and place tiles from any of them on the tilemap canvas. This way you only need 1 tilemap object per layer and can use any tiles you wish, and you can use Tiled & the TMX Importer without having to use sprite objects acting as tiles.