I really don't think we have to worry about Nina too much - given that she's one of only 4 characters that's been in every Tekken title so far, it'd be beyond ridiculous if they didn't include her eventually.
On a side note, while I'm aware that I may well be the only one that actually cares, but... does it bother anyone else that some of the infinite stages still have that 'background isn't at all blended with the foreground' problem, to the extent that you can clearly see the line that cuts off the fighting from the background?
It's odd, because prior to Tekken 5, it happened on EVERY infinite stage, basically, whereas from Tekken 5 onwards, it only happens on some of them. So they're clearly capable of blending them properly, but just... choose not to half the time? :/