I don't think they can really make it a Zero sequel. UBW doesn't really deal with Ilya, Sakura or Kotomine in any significant manner, which means that a large chunk of the Zero side-plots will be left out (and, as a result, Rin's character development is also incomplete). And, I don't really see how they can add any of those things to UBW without significant changes, which are unlikely given that all three are covered in HF. The HF movie is likely to be far more of a sequel to Zero than the UBW anime.
There are elements that tie in from Zero in UBW. Mainly Kiritsigu's dream and the continuation of it as seen with the Shirou and Archer paradox.
Yeah, there are elements of Zero that are covered in UBW (which is true for all three routes), but I think that there are a lot of elements that
aren't and can't really be.
Not to mention, there have been theories elsewhere, that in UBW they might have Rin being the one who kills Kotomine. Which would give closure to Rin on that front.
Honestly, I find that unlikely. Rin killing Kotomine would necessitate changing the story around somewhat, given that she was tied to a chair at the time in the original VN, and I don't get the impression that they intend to make significant changes to the flow of the story.
And, whilst UBW Saber is perhaps "closer" to Zero Saber than Fate Saber is, I don't think that's really the point. The point of Saber's development is for her to go from "I'm a total failure of a King who needs to sell myself into eternal slavery at the hands of the world in order to erase myself from existence" like she is in Zero to "actually, yeah, I should just accept what happened and move on", and that development isn't so clear in UBW as it is in Fate.
The development is still pretty clear in UBW. The problem with Fate is that the "romance" just completely defangs Saber. In UBW she isn't defanged. She makes the realization through Shirou and Archer. She finds a kindred spirit in them and through their answer she finds their own. Which also ties back to the Saber and Kerry clash in Zero.
Yeah, I guess. I think a reasonable amount of Saber's character development is glossed over in UBW, but that's mostly because it was covered in Fate already. I wouldn't be surprised to see them add stuff about Saber's past as extra scenes.
Sure, admittedly, HF is more of a "continuation" of Zero (which is probably why they're making the HF movie or movies), but UBW is a continuation for Kerry's ideal.
Well, both UBW and HF act as a continuation to the "Kiritsugu's ideal" stuff, in different ways. But, honestly, UBW Shirou doesn't take up Kiritsugu's ideal. If you dumped UBW Shirou in HF around the time of MoS, I don't think he'd go down the MoS path, I think he'd give Sakura a chance. UBW Shirou accepts that he might have to kill sometimes, and that he can't save everyone, but he doesn't ever take up Kiritsugu's purely Utilitarian attitude to people. He still
wants to save everyone, and I think he'd be very reluctant to kill someone who wasn't acting to harm others of their own free will.
Very excited for HF
Yeah, it's good that we're finally getting a HF adaptation. I just hope they don't rush it like Deen did with UBW, though. I personally think it needs ~6 hours to do it properly, but a little less might be OK if it's done well.