Many tools offer offline file browsing and full text search. Anything from midnight commander to Alfred to ripgrep.

I think the valuable and difficult thing is high quality, fast, offline semantic search, including indexing (so my files don't go to your servers -- that's an immediate no-go for all the important industries).

I've tried my hand at this a couple times and it's hard! But really hope you succeed, because it would be a big game changer in how we organize and use information. Every company out there would buy this immediately.