MS:
Halo: 343i are a very capable studio, but they are no Bungie. Also, long running series, franchise fatigue, meta already on downwards trajectory, last meta 87
Gears: Will be made by a new studio, and again, meta already creeping down throughout main series (94-93-91)
Forza: Think this is the most likely series to spoil my prediction, but again would not be surprised if it was in the mid to high 80s instead of 90s this gen
Sony:
Uncharted: Last game receieved 92 metacritic, but I don't think more open levels, and better animations and A.I will be enough to stop this entry from falling below 90. Reviewers will be much less impressed by the series' trademark spectacle and set pieces. And from what we are hearing, this will be the last in the series, which increases the odds the franchise won't see another 90+ score.
God of War: Meta score creeping down (94-93-92) for main series. Time period would definitely have to change in my opinion for game to possibly see another 90+ score. Otherwise it just won't be fresh enough.
Gran Turismo: Last two entries have been mid and low 80s. Nothing would suggest an upward trend at this stage, given how far apart releases are and how slow the series has been to react to changing expectations.
Realise this isn't that bold a prediction, but interested to hear what others think. The reasons above and the generally harsher reviews games deemed similar to last gen offerings have been receiving makes me think there is a good chance this prediction may turn out correct.