The stable system was removed. You'll notice there's no stable boy any more.
You can already tie your ox down in the barn/stable, and even lead it into one of the stalls and shut the gate. The barn would get very overcrowded if many more people decided to use it, though, so a bigger barn isn't a bad idea.
The problem I see is that people are generally rather lazy, so they park their oxen wherever it's convenient. That generally means right on a well-traveled path (like the gap between the fence and windmill in front of the Hlint craft hall) or in the craft hall or smithy. Even those taken to the barn seem to end up stacked just inside the door. So, as nice as a bigger barn interior might be, I doubt it would see much more use than the current one because of the desire for convenience (read: laziness).
(Yes, I am rather pessimistic about people and their oxen.)