This might only be useful to people who read a lot of books at once, but I think it would be neat if somewhere there was a stat that showed up on your currently reading page that had the total number of pages for all of the books you're currently reading, and then how many you'd read and how many pages you had to go. I'm kind of thinking of an overall percent read for all the books you're currently reading.
I can see this getting applied to tags, too, with a percent read number for each tag.
Nothing urgent or life-shattering, I just think it would be neat.