Add an "Already owned?" option to the store advanced search engine.
I think we all know that Steam's options for searching your library are woefully inadequate compared to the rest of the site -- you can only search by title and that's it; no support for genres, tags, or anything else. There's been a lot of suggestions to improve that (and I definitely think those would be good.)
But really, there's already a somewhat-better search engine in the store. Why not just add an "Already owned?" dropdown to the store's advanced search options, so you can choose to limit your search to games you already own (or games you don't own)? With that, users could leverage the versatility of the store's search engine to search their own libraries.