I think there should be a block that can tell you what object is currently your canvas, as this would make it much easier to make a block that, for example, edits a costume using the pen. This is technically already possible with the blocks we already have, but there is no way to actually go back to the previous canvas as there is no way to know what canvas you were on before changing.
Another thing I would like would be a way to use the alternate menus that are currently restricted to primitive blocks, and that makes it so that you can’t, for example, set a sprite property in a custom variable block.
Something I would also like that’s related to my last idea would be, at least in an extension, a way to call the scripts within blocks that set things like the menus so that you can read the menu items that are actually show up when you open the menu.