Well I had some thoughts about it...
As I understand it, it would implement a complete rewrite of gallery.
But maybe I am wrong, so if someone is very familiar with the attachment system please correct my thoughts.
1. Either images would have to be stored twice, (once in filesystem and once in attachment system) otherwise they would not be backwards compatible.. (a no no for big galleries.. webspace 30 vs 60 gigs)
2. OR they would have to be moved completely to attachment system. Now I have a bad feeling about some galleries, (i got an example of one with 390 categories and over 600.000 images) as vbulletin by default stores all images in database. Of course you can move them to file system, but if I am not wrong, vbulletin changes the filename to something completely different.
The solution for me would be to have sort of "softlinks" in attachment database, dummy images with no image data. so this so instead of pulling the stuff from database or vbulletin filesystem, it would pull the images from gallery filesystem. and just have the full links in attachment database.
I admit I do not know if this is possible. If there are hooks in the appropriate locations. I will look into it, but only after 3.0 gold is out.