This is interesting, I was building something similar for my personal use case a while ago but there I have to build a plugin for each website I want to support which is not ideal (https://github.com/dewey/feedbridge). I'll have to check it out, community maintained integrations make a lot more sense.
There is RSS-Bridge which is doing exactly that: A plugin per website, common libraries.
A pragmatic approach. Trying to build a generalized framework handling each and every edge case promises endless complexity.