The Romulans are on the cube, no doubt salvaging and studying the technology, so its probably both a Romulan and Borg story.
The countdown comics said the Romulans used Borg technology to refit the Narada into what we saw, so that cube could be the source for that tech.
Given that there's a comic trilogy tie–in called Star Trek: Picard – Countdown coming out prior to the series, (as well as a novel,) this seems like it's almost a given. You don't do another "Countdown" comic if you're goal isn't to reference the first one. Given that the novel is called Our Last Best Hope, a nod to one of Abraham Lincoln's speeches, and it appears that the Romulans are enslaving former Borg Drones, I would easily believe that this series is going to be a Romulan and a Borg story. Most importantly though, Picard's two most life–changing incidents involved the Borg (Assimilation) and the Romulans (Death of Data,) so having to confront both of them simultaneously seems like the perfect scenario for Picard to deal with.
I actually like what Im hearing story wise... and this actually sounds like it might make a great movie... maybe even a great trilogy. In that spirit Id say maybe they have 5-6 hrs of good story telling... this should really be about 6 episodes... my concern is that to fill 10 episodes they will do what a lot of net shows do and draw it out with poor writing. This has been the bane of both seasons of DISCO... though admittedly the plot seemed to wander a bit through both seasons... hopefully they nailed it all down and write it in advance so it doesnt suffer.
I believe the plan is three seasons if the first one does well based on comments Sir Patrick Stewart has made, which is how you'd get your Trilogy. I'm actually not concerned about the ten episode count. There's a lot to pack into the first season alone, (just look at that trailer, there's so much to tackle there that I'm almost worried that 10 episodes might not be enough to cover everything they need to without rushing some of it,) that I doubt this will drag. DSC has suffered because its writers haven't used their episodes efficiently. The first three of season one were absolute disasters, and I'd go as far as to say virtually everything outside of the mirror universe in season one was a disaster with the exception of the Harry Mudd episodes, one of which was effectively a standalone episode proving that the plot for that series might have been better with less serialization rather than more. Season two's problem was the revolving door in the writer's room; it's so obvious if you watch DSC S2 again that what they ended with clearly wasn't what they started with. Early on it looked like the Red Angel was going to be some sort of myth that Michael had heard of and forgotten about. Then, as the show got further in, the whole plot with Control sort of came out of left field and the entire season turned into something else. For awhile it felt like they forgot about Terralysium, before referencing it again almost in a "hey wait, we have this thing we need to address" type of scenario. This bugged me because they could have moved things a long a whole lot more smoothly and had plot points feel more natural instead of seeming forced. I don't think "Picard" will have this issue though, they seem to be handling it differently, and Kurtzman himself has suggested he wants the pacing to be different from DSC, more in line with TNG, but also not outright identical, which is good given that Picard is clearly meant to be different from TNG.
I would love to see this series really outdo DSC. If it does I think we could see this time period get the focus it deserves going forward once it becomes obvious to the studio that the fans who've been clammoring for something like this have been right all along and that going forward from here is where Trek's future is. (Then again, I also think DSC needs to spend no more than a season in the 33rd century and should ideally wind up in the timeline of Picard when the time comes to wrap DSC up.)