I agree that SAB capacity would be a potential issue if YC had a DVC although I suppose it depends on the size of the property. I can't imagine it being that large just do to the constraints of the buildings currently there. If it ends up being fairly small, it likely wouldn't impact things a great deal.
I agree that there likely is more room to add on at BC than YC and Disney may consider that if they consider the properties to really be one entity. I think it may be kind of a mess having an "old" and "new" BC, though. Even if they offered contract extensions on the current property, not everyone would take it so you'd have contracts expiring at different times (like OKW) which I don't know if DVC would want. Then, you have people who would only want to stay in the old or new section, based on preference. Nothing would likely be a major issue but minor complicating factors. It still may be easier, though, to have one larger DVC property rather than 2 smaller ones. Frankly, I'd be happy with a YC or larger BC DVC but if either comes, it likely won't be for quite some time.