You can't really get a lot out of polishing bitmaps, though, unless you want to basically retouch everything by hand, which is an extremely time-consuming process.

The only realistic way to increase graphical detail would be to re-render the backgrounds from the 3D models they probably already have lost. A bit of a shame, though. If they had them, and managed to find a way to automatize the process, the actual rendering of those scenes would probably take very little time on modern hardware.

Even re-rendering all the backgrounds manually would probably take less time than retouching all the bitmaps by hand. I just doubt they find even that worthwhile.