The biggest issue with the MGS Rising engine (which has to be recoded from scratch) will be getting it to run without installation due to the 360 version.
That's probably the real reason why it's being re-tooled.
Visually, I'd expect it to be an incremental improvement in visual quality, similar to the difference between MGS2 and MGS3.
But seeing as how Rising probably won't be ready for another 2-3 years, the bar for visuals may be raised even higher by then.
What will amaze me is if the actual visuals on my display for U2 are significantly better than Uncharted, given the relatively short 2 year span between games. Unfortunately, you can't tell the whole story from web screen shots and hand held video and the U2 demo didn't exactly showcase the visual improvements being that it was only a multi-player match.







