Yeah, so one thought I had had was to press flowers and plants and try to incorporate them into covers for decoration or into homemade paper. However a friend pointed out there might be some problems with decomposition, maybe (I can see the potential at least) and I'm not totally set on how to prevent that. I think I would need a heavier finishing gloss, but that might benefit me as it is too.
I was going to mention. Glue. I have so much and so many kinds of glue. Modge podge, white elmers/school glue, glue sticks, hot glue gun, basically any basic fixative option. I've been considering trying out a spray adhesive as well. There is so much glue in the world! And it is all really different and it matters!
I've also experienced with a book where i covered it with essentially a mat of ribbons I had wove into fabric. It looked really cool and had awesome texture but the hassle was not worth it. Not pictured cuz I gave it to a friend. In that vein I've pondered knitting covers (probably too bulky and time consuming but cool texture) or maybe trying to incorporate cross stitch or embroidery into covers. I'm mostly coming down on the side of "going to take way too long to pay off" on these, but who knows if I still have thread and needles I can brush off at my parents' house just to experiment?
I am always trying to think of new ways to do book covers. I think there's probably a lot you can do that would be really fun.