It wouldn't be Horizon Online since that project hasn't been cancelled.
Given that Days Gone 2 was already set to be powered by Decima according to this LinkedIn profile, it wouldn't be weird to see "Unannounced Title B" -- aka, their cancelled new IP -- also running under that same engine (as reported by David Jaffe literally four years ago during one of his streams back when the Garvin/Ross/Bend drama was hot news) too.
Then "Unannounced Title C" would be whatever they're working on now.
Sounds right. They almost didn't work in Days Gone 2, but maybe one of the few prototypes could be a test to migrate part of its base to Decima. We have to remember that Bend did work in Horizon Forbidden West as support team.
Regarding their new IP with live service game elements, we saw job offers mentioning it was Unreal Engine 5. But again, they could have also made tests with Decima.
NuSony will literally spend almost half a billion dollars on some shitty online slop than invest on a game that has an actual following and a huge potential to be a long running franchise. People say games get cancelled all the time, sure but Snoy are aware that Days Gone has a huge cult following, sold really well, and fans have been craving for a sequel. It doesn't make sense for them to not to invest on it, unless they don't want to fund a game that isn't a "woke" slop. Fuck nusony
The Bend Studio internal boss was the one who decided to don't pitch Sony Days Gone 2, and to pitch them instead the new IP. Sony greenlighted it, which was the first thing Bend sent them after releasing Days Gone 1.
Sony also greenlighted them Days Gone DLC, PC port, movie adaptation and remaster. In addition to this they included it in the PS Plus Collection that PS5 had in its launch window, and was like a 'best of PS4' collection. Sony wouldn't have done these things if they wouldn't have been happy with it.
And yes, there are games cancelled all the time, and also games that -like Days Gone 2- never get greenlighted to start their development and end being just a powerpoint, a few design docs and a handful prototypes.
Now that their new IP with MP elements got cancelled, pretty likely the next Bend game will be a SP only sequel to play it safe. So I'd say pretty likely they'll work on either Days Gone 2, an Uncharted game codeveloped with, or supported & overviewed by ND.
I'm always surprised how MM survives.
They survive because all their games have been profitable and won many awards with all of them. Plus got extra money from externally developed LBP games and content.
They are a relatively small studio and a huge portion of their workers were hired as their first gamedev job, as juniors straight from university or from the community. So it's a very studio to mantain and achieve profitability from their games compared to most of the other Sony teams.