I like to keep on top of new messages by using the Atom feed from all the boards on the forum. Unfortunately, the posts of different topics appear interleaved, so it is hard to follow old, or not very active, discussions. Atom supports a threading extension (RFC 4685) that seems simple and easy to implement. Something like all posts being under the OP.
have you looked at https://www.simplemachines.org/community/index.php?topic=25009.0 ?
It is sadly neither easy nor simple to implement in the current structure.