Bit out there, but as per the title I've found myself thinking that the primary use case for the placeables GUI (needing to place one or a few of something that doesn't come up often enough to justify putting on the hotbar) would be improved quite a bit if it wasn't cluttered with all the things that are already on my visible hotbar.

I'm not sure to what extent the APIs exist to support this, but it'd certainly make it a lot more useful for me. Often when I make a new machine type it's because I want to use it right away, and while the placeables list is usually quite a bit fewer unique items than my entire inventory it'd still be a lot more convenient if it wasn't also duplicating the selectors for belts, power poles, furnaces, pipes, etc. that are right next to it.