-I would like to be able to select a group of tiles and then when I use the "Fill Tool" have it fill using random tiles from the selection. For instance I have 8 different seamless grass tiles and I would like to be ably to randomly place them to gave a more varied look.
Something that would facilitate this has been proposed, the idea of tile "libraries" which are a group of tiles selected from a set. There is the custom brush, which is a spray brush and can "spray" tiles at random, but this only handles one tile at a time. That's really a cool idea you have.
-Be able to flip and rotate tiles
This was added at one point, and then removed. The reasoning behind removing it was that if the tileset came from a single image, Tiled would either have to save new images for some or all of the tiles, or it would have to reconstruct the tileset image. Both these solutions we felt strayed too far into the realm of a graphics application, which Tiled expressly tries not to be. (as opposed to TileStudio)
-I would like to be able to select multiple tiles in the "Edit Tileset" window and add/set properties to all selected tiles. Currently it only changes the first tile in the selection.
I've also wished for this one.
That's also not a difficult feature to implement.
All in all, I think all three ideas are cool. I can't say really yet which will actually make it in. But keep ideas coming- it helps stir some inspiration and maybe some drive.