Ubisoft's game subscription service, known as Ubisoft+, could be on it's way to Xbox Game Pass at some this year according to various rumours.
While nothing is confirmed, so take this with a pinch of salt, there is a strong chance of the rumours being true, at least according to Windows Central, a trusted source for Microsoft news. If true, the service would be coming towards the latter end of 2021.
I've seen rumours about Ubisoft Uplay+ joining Xbox Game Pass Ultimate.— Jez (@JezCorden) December 30, 2020
I think this has a strong chance of being true, based on stuff I've heard.
It wouldn't be the first service to do this either, with EA Play already being a part of Xbox Game Pass on consoles. It seems likely that this would only be a part of the Xbox console version of Xbox Game Pass, and not a part of Xbox Game Pass for PC.
Currently, Ubisoft+ is $14.99 per month on its own, so would be a significant value add to Game Pass. EA Play is heading to Xbox Game Pass on PC at some point soon, though, so it's possible Ubisoft+ may do the same in the distant future.
Of course, this is all rumour for now, so take it with a pinch of salt, but it would be a pretty welcome addition if it does turn out to be accurate.