I guess this comes up kind of in response to the google thing. Honestly I don't know why people keep using anything from google. About two years ago I decided I would never use a google product again, and I never looked back.
For RSS, I have been using Liferea since then, and I totally recommend it.
RSS is a good example of an area where having an application be backed by a service brings huge value to a lot of people. If you want to access your feeds from multiple different machines, synchronization becomes awkward with a purely local app. It gets even more awkward if you want to do it from multiple different platforms.
You could use some commercial non-Google service as the backend instead, but those could equally well get cancelled.
You could maybe host your own service and have full control, but the installation and maintenance is a huge time sink. And in both cases the amount of applications you could choose from is likely going to be smaller than with Reader.
There can also be other benefits in sharing the service with other people. For Reader, the infinite feed history was a biggie for me. It really was quite amazing to subscribe to a 4-year old podcast, and have easy access to all the 200 podcasts from that period even if the current feed only showed the 10 latest ones.
(I used Liferea for a while maybe 10 years ago, and even contributed some code at the time. But ultimately even the web-based RSS readers of 2005 had a better workflow than a local RSS reader. Sure, the UI sucked and you were completely at the service provider's mercy. But at least it was possible to access your feeds both from work and from home.)
Liferea is worse than useless for me, it's downright dangerous: I procrastinate enough as it is, I don't want an RSS reader than I can only use when I'm at my computer and most probably should be working! RSS readers go on the tablet or phone to check in my spare time.
Liferea supports Tiny Tiny RSS sync, so that could be it. You can host http://tt-rss.org/ and use Liferea as desktop app (and may be the web interface for mobile).
In fact that's the only part that I really liked of Google Reader: it was the perfect backend (I got used to the web interface, but the real experience was with the native apps).
It would be nice to have a "standard" API for RSS sync that could be implemented by servers and clients. In that way we could host our own RSS backend in the same way we run a SMTP and IMAP servers to have our own independent email infrastructure.
Tiny Tiny RSS API could be a good start, or we can agree on a Google Reader compatible API.
I've used liferea with google reader for some time to keep things synced between computers and devices. I really hope some of those projects to keep that API alive bear fruit just for that. Though I have seen liferea become non-responsive when having 1000-2000 feeds to check (like other places, i just add stuff and never delete, because there's too many to sort through...)
Sage is still around? For a while, it was stagnant and then was sort of forked. There were some rendering/vulnerability issues that were not being addressed in a timely fashion (in some cases in one fork, in other cases perhaps in both forks).
Do you know enough about it to briefly describe where it is at, now, and/or which fork (if still forked) is better and why?
They are just jinxed that way! Do it all right, but just not getting the mindshare that they (in a way, rightly) deserve (despite being closed-source)!
And the browser itself is just fantastic in terms of speed, configurability and performance (despite failing on pages here and there due to no fault of theirs).
Opera Software recently took the decision to leave the old browser-foundation behind for Chromium[0], which may change the things you listed for better or worse. Since Chromium doesn't support RSS natively, it is currently unknown whether it will be supported in future versions of Opera.
Opera are only replacing their rendering engine with the one used in Chromium, they are not using Chromium to replace their browser. Opera's RSS feature doesn't even touch the rendering engine and is built entirely with their native widgets - but even if they were to put it into the rendering engine, RSS is just XML, and can be styled in any way they want using standard web technologies.
tl;dr: The move doesn't affect Opera's ability to put RSS in their browser.
I have refrained from making my mind up on that particular development so far (Why webkit?, why not gecko?, this is baaad!, no this is goooood!, blah blah.....etc...). I am simply going to wait and watch that particular development as it unfolds, TBH.
For RSS, I have been using Liferea since then, and I totally recommend it.