Do you realize that shoehorning HF, Fate and UBW into one mix will never end well? Fate/UBW are more or less a Holy Grail War. HF is a failed one, with drastically different elements being in the foreground, and different themes. Otherwise, we can get another DEEN anime. Without bondage Sakura, but not much less awkward.
Yes, I'm well aware of that, but it doesn't mean they aren't going to try it anyway....
I can't see them not covering HF in some way. Ignoring Sakura would leave a bad taste in the mouth of people coming from Zero, and Rider is a popular character so I suspect she'll be given some importance in the story beyond "generic mid-boss".
If Sakura is mentioned in more than passing way like in Fate and UBW, it will be likely about Rin and others saving her from Zouken, but that won't be a major plot. Likewise, Sakura's plight was one of many subplots in Fate/Zero.
Sakura's plight was a pretty significant part of Zero, I can't imagine it just getting ignored. And, if Zouken is dealt with it makes involving Sakura more generally a lot easier.
I agree that a direct HF adaptation is unlikely, but I don't think that any Sakura-related segment will be minor and I suspect that HF will at least be alluded to.
1) Fate/UBW accomodated to fit storylines mentioned in Fate/Zero. Sakura gets more than cooking scenes and harassing Shirou with Taiga about the newcomers, but nothing to an extent of HF.
How would that work, though, without making it a major part of the story? Zouken ain't easy to kill....
2) A new route. Ilya route, Taiga route, Archer route, name it. Likely an official 4th route since Nasu is supervising the script, unless he nods at everything too busy playing Dark Souls.
If they do an alternate route, which is possible, I don't think it'll be focussed on any one character in that manner, especially not a new one. It'll be a more general thing focussing on
everyone. There's no reason why the route has to focus on one particular character, or why Shirou even needs a romantic interest (or why the romantic interest can't be one of the original heroines but in a new route).