A long time ago, I heard about those dedicated 2ch readers. IIRC, they work like newsreaders (USENET) did, scanning boards for posts, and only downloading the new ones.

If something like this existed for english-language board software, I believe it would be much easier to keep up with all of the tiny niche image and text BBSes out there. Instead of having to check a dozen tiny boards for new posts, they would simply appear at the top of your "new post list" any time one was made. Or, if you only wanted to follow one thread, you could do that as well. Since the server would do nothing but send you the posts, you could easily perform all kinds of searches and content filters and whatnot on the posts as well.

When updating, the program would send the server the number of the last post downloaded. The server would then check if any newer posts exist (whether that be in one thread, or in a whole board). If they do, it would send them to the reader in plain text (not a whole formatted HTML page, just the post data), perhaps using some standardized format like JSON or whatever. Lastly, the reader would replace update the record of the last post downloaded, and the update would be done.

This is very much like what you are describing, but since it is operated on the client-side instead of the server-side, it would be a lot more usable for the, well, users, and a lot less of a burden towards site operators. In fact, in the same manner that is does with newsreaders, it would actually require a lot less bandwidth per user in the long run, given all of the duplicated data sent when a thread updates.

I'm aware that we have RSS feeds, but those aren't quite good enough. MrVacBob added a JSON interface to Shiichan, but no one (the last time I checked, which was over half a year ago) ever did anything with it, other than progscrape, and I'm pretty sure that was a CLI program, not something comfortable to use on a regular basis, and not all that close to what I was thinking of.