Carrying around eight Pokemon instead of the usual six would be an interesting change for them to make, but I don't think it's really necessary. Part of the strategy for team-building comes from the fact that you only have six slots to work with, and really I don't think you need any more than that; it's been good enough for the last decade and a half. There are changes for improvement, and changes for the sake of changes, and I think that would probably fall into the latter category.
Triple battles and Rotation battles are already a complete and utter mess though, why throw a fourth Pokemon into the mix and make it even more chaotic? I really hope they don't do this in any format, because it'd just be a complete and utter nuisance. I never bothered with triple or rotation battles in the fifth gen unless I was forced to - and then I grumbled about it whilst trying to get out of it as quickly as possible - and I can't see myself enjoying 4 vs 4 battles either.