I've been mulling over some mix of the two ideas for some time now. I keep thinking it's been done or will be done soon, but I've not seen it (or... am I missing something).

Basically, some form of aggregation/curation of individual blogs. The 'planet' aggregators had an initial vision of this, but they were 'read only'. It would be far more useful if I could engage with each individual blog from the main aggregation site. Authors keep control of their own content (including comments, probably), but are aggregated/federated out to larger networks, but visitors can engage with a variety of topics without having to go register/join each site.

Has this been done and I'm just missing something?

I'd probably pay a nominal fee for a service like that, assuming that a portion made it back to the sites I engaged with (reading or commenting or sharing or whatever).

If medium was a collection of blog posts from existing blogs, and the comments were shared between medium and the host blogs, both ways, for example. Anyone doing that?