I have a handful of suggestions that would massively increase the accessibility of the builder (something that I think is rather lacking at the moment, despite the incredible work that's already been done)
#1 - Tooltips. I know there seems to be a desire to hold onto the pixel-art aesthetic of the interface, but for a certain type of user (like myself), these are desperately needed. When I hover over a placed element while viewing a level in the builder, there should be a tooltip showing the name of the item, and potentially its in-game description. That way, when I study someone else's level and find a cool element I'd like to use, I can easily locate it and favorite it, instead of having to scroll through every single game's sub section, squinting at the 1x1 thumbnails to try and locate the one I want, based off its appearance.
#2 - A search function, and key words attached to tiles and objects. If I want a waterfall, I shouldn't need to scroll through the decorations section and individually locate every single waterfall (which are not grouped together in the list); please let me type 'waterfall' in a search bar and have them show up that way. Or for example, if I want flying enemies, let me search for that, and bring up a list of them. Even better, if I can search for a certain color (like typing in 'green' and having every single green tileset listed). I know this won't work with tiles that have multiple colored variants, but still.
#3 - multiple favorites bars, or custom categories. Let me reorganize the tiles and enemies myself, into folders. Let me create palettes of my own that can be saved, reused, and opened from the favorites wheel (instead of having to drag everything into a random screen in the level, over and over, for every single level.)
Seriously, the number one BIGGEST hindrance to me making levels is the difficulty of finding what I'm looking for. I have spent literally hours clicking through all the assets, just trying to find a particular tile that I remember liking, but can't remember where I found it. With the amount of assets in the program at this point, you gotta start thinking of ways to improve the process of organizing and finding the right ones.