Yeah, I think making it more HF-centric whilst giving Saber a larger role is plausible. I think you can tack Saber and Archer onto HF more easily than you can tack HF stuff onto Fate or UBW, because there are a lot more ways to involve Saber and Archer and their backstory than there are ways to involve all the stuff about the shadow.
But, at the same time, making it HF-centric does mean Saber's role will be reduced. You can change the story somewhat to prevent her being turned Dark, but I don't think you can really cover everything whilst following a HF-like route. I'm not sure that Ufotable will want to do that, honestly.