This is one is kind of self describing, but I have a lot of collections and custom blocks and all of the deluxe blocks and I usually only want one or two blocks from any given collection -- it would be great to be able to right click on a block and "mark it as a favorite" (maybe with a tiny little 50% alpha heart in the corner?).
And then have an extra collection listed at the bottom that's called "Favorites" that just has all of your favorites listed there. -- So you could build a palette of the blocks you plan on using.
Alternately (or in addition to) if we could have a collection named "Currently in use blocks" or something similar, that just showed the different blocks that were being used in the current file that's open, that could be just as helpful, and also make editing files easier.