For really large maps it would be nice to be able to scatter misc doodads (grass, flowers, etc.) over a large area.
Would like to create a very large map (500 x 500) and then be able to break it up into smaller maps (25 x 25 or 50 x 50). I envision this being a save as feature. I set the width and size and Tiled will break up the map into whatever size I specify. It will also preserve the layers. 
Right, this is currently not possible. I guess it would require the introduction of an airbrush tool, since I’m not sure how it would fit in with any of the existing tools. Eventually there will need to be a place for tool-specific settings in the UI.
Yeah, this could be some kind of export mode, but there would be a number of configuration settings to take into account, like what to do with objects that fall partly in several of these smaller maps, or whether to have some overlap at the map borders. As such, I guess this kind of thing is better solved by making Tiled scriptable, so any kind of cutting could be automated (along with enabling many other possibilities, of course).