I don't want a thicker rMBP. I would prefer that it get even thinner and lighter, so that traveling with it is easier. I think there is a group, possibly a large group, of people who use rMBP's that rarely, if ever, use them on battery power.
Maybe Apple should produce a version with significantly less battery in it, maybe enough for 4 hours of use, then take the space and add in desktop class CPU / GPU? Since so many people use laptops as desktops nowadays, with just the occasional off-plug use, Apple could offer Mac-Pro-like power in a laptop.
And given the move to the USB-C ports, those that need more battery power can easily use an external battery, making less need for huge on-board batteries.