Ok, just finished a sizeable project so I've come up with a small list of things that would be handy or nice (and one possible bug).
Just another tweak to the tile tool layout, aside from being a grid it'd also be really useful if you could rearrange them, so you could group together say, single use tiles or multi use textures just to make searching easier in a big project.
Being able to have more than one avatar design, maybe linking different ones to different rooms as is currently the case with colour palettes? This would be great for creating games with more than one character or more than one mode (eg, flying a plane across a map, and then walking around different locations on the ground).
A sprite being able to have more than one piece of dialog, either so a player can return to a sprite and read something different, or just to break up very long pieces of text (I've had trouble in a couple of Bitsy games where I have accidentally touched a sprite for a second time and gotten stuck scrolling through a lot of text I've already read).
If you hit "Enter" when using the dialog box it'll put a new paragraph in, but everything you write afterwards will be deleted once you play test it. I understand if it's not possible to put new paragraphs in, but it'd be better if this just didn't register than deleting your writing, it's very easy to hit that button out of habit.