With The Pirate Bay in ruins, and with its founders denouncing it for all that it has become (ridiculous porn ads and profiteering), one wonders what a replacement that doesn't suffer the same fate might resemble.
While leafing through Hacker News it occurred to me the Bitcoin blockchain is already an ideal mechanism for publishing a completely decentralized BitTorrent index.
Since you can easily fit the hash part of a Magnet URL in a Bitcoin transaction's OP_RETURN value, all required to publish a torrent is to move its description (title, description, categories, magnet URL) into a torrent of its own, since in the general case it would not otherwise fit within a single Bitcoin transaction. Indexers find the first hash via the blockchain, use it to fetch the full torrent description via the BitTorrent DHT, which in turn allows it to find the torrent via the magnet URL (or alternatively, includes the .torrent file directly, but this trades network efficiency for hosting overhead).
To accelerate bootstrapping new indexers, occasionally "roll up" descriptions could be published, which are just torrents that aggregate a large number of descriptions (bucketed say, by date, DHT swarm size, or similar). Add an identifier and a dollop of public key cryptography to these roll-ups, and you effectively have a trusted "channel" - one guy or group with editorial control over the roll-ups they publish, and magically you have something very close to ThePirateBay's database again, only this time no load balancers to attack in a single data centre in Sweden, and no single web site to serve it covered in porn ads.
For this to work, indexer software would need exist that knows how to scrape the blockchain, fetch the description torrents and build a searchable full-text index, which might be presented via a web interface (say, something that resembles The Pirate Bay). Since scraping the blockchain is fairly storage, computation, and network intensive, probably there would be relatively few indexers, although a dedicated user could always host their own.
For resilience, ideally BitTorrent clients could be modified to help host the description torrents, otherwise their only copies would again be concentrated on the (relatively few) indexers, where they could easily be attacked.
Yet another alternative is to produce a standard 'torrent description' file to include in the source torrent itself. That avoids having to modify the BitTorrent clients to contribute hosting (since the 'description' torrent becomes the torrent itself), but prevents bootstrapping such a decentralized system using the millions of torrents that already exist.
Probably this idea isn't new. Still, it's fun to think about, but likely weeks to implement!