[ColinWright]

#1 already exists in HN Notify

I asked about #2 recently and the only responses I got were strongly negative.

#3 exists - I think it's actually called HN follow, although I don't use it and can't remember exactly - you could do some searching.

Many people have produced versions of #4

#5 seems under-specified - everyone you ask will think you mean something different.

As a single data point, if you build it, I wouldn't use it.