There are several websites with ideas for things to build with spare parts. Several YouTube channels too. You might try browsing these if you are looking for ideas.
This guy's channel has some amazing constructions: https://www.youtube.com/c/JKBrickworksVideo
I believe I've seen him mention that Lego has accepted some of his ideas for sets, if that tells you his skill level.
I fancy this one:
Simple yet impressive to look at, as tensegrity structures always are. I haven't been able to find the parts I need for it from all the spare parts my kids have so I may get around to ordering them one of these days.
I've already got the plans for what I'll be building, I purchased them online. There will be a big reveal...if the set works. Because I'll not mix my sets up I have to find new designs that utilise only the one set. Most of mine have more than 4,000 pieces so are usually reworked into some pretty cool things.