I’m kind of excited about payment request, would that integrate with e.g. Apple Pay? Reducing the overhead to paying sites is one of the things that I think could turn the web around. I have wished that e.g. Firefox would put a “$1” button on their toolbar, which would allow you to just give the site a dollar. Practical problems aside, could really improve the best parts of the web.
PaymentRequest does support Apple Pay and is also supported by Google, Samsung and Microsoft at least - so building a PWA with in-app purchases is very much possible now
As a side note, I actually built such a browser button that you mention when I was at Flattr + investigated ways to identify the rightful owner of that page so that they could claim the promise of a donation. We never got it to fully work on all sites, but it worked for some of the larger silos, like Twitter and GitHub, and also worked for those who had added rel-payment links to Flattr, but we/I investigated having it crawl peoples as well public identity graphs to try and find a connection between the owner of a page (through eg rel-author link) and a verifiable identity – like their Twitter account or maybe some signed thing Keybase-style. That ended up with the creation of https://github.com/voxpelli/relspider but the crawler was never fully finished (eg. smart recrawling was never implemented) and never put into production. I still like the idea though.