Or: what would it take for everyone to have part of a copy of the index, according to their capacity / needs / etc., and for them to be able to easily search th

Or: what would it take for everyone to have part of a copy of the index, according to their capacity / needs / etc., and for them to be able to easily search the parts of the index they don't have? To pre-empt the "you've just described BitTorrent" comment - only in the vaguest sense; you'd need search functionality on the chunks themselves, and ideally you wouldn't have to copy (or even stream) your peers' search index chunks to search them. I guess there's a trust / security issue here around "search index poisoning"; to resolve that, you'd probably have to lean on SSL verification and all of its attendant infrastructure for now.