I still don't know what makes you say redoing 2D doesn't make much sense - take a look for example on Monkey Island 2 Special Edition:
http://www.videogameszone.de/screens..._edition_3.jpg
2D does provide incredible detail support, spectacular light effects and animated elements of background etc. And requires less effort than doing 2.5D which, let's face it, wouldn't look as nice (and/or work smoothly) as prerendered 3D graphics. You'd be able to create game which looks good and still is really close to original...
...without huge system requirements