Yes, those things could be done, but you're still asking software to make decisions on what is "relevant" and what isn't, and that is pretty hard. Billion dollar companies are made on just that.
Either way, we're not adding or going to try to add any of these features. Just fix your settings or subscribe to fewer threads. I managed to go for 5 years without subscribing to a single thread. You can too.
Likewise, I too hate it when people post just for the sake of posting, without contributing to the topic. In fact, it's worse than not posting anything, because it's buries the good posts under a bunch of junk. But trying to make people stop doing that is just as impossible as what you're asking for. It's also pointless as people will just do it anyway.
(If you really want to piss me off, make 20 or so identical, useless posts in the introductions-section. That will fill up my "new content" view with absolute junk.)