I don't think you'll see a bay capable of supporting an optical drive -- they're gone, and few people care -- but I'd still expect the next generation MacBook Pro 15" to augment a cooked-on SSD with space for a user-fitted 2.5 inch HDD. And the 17" model should have space for TWO user-fitted 2.5 inch drives, as befits the model's target market.

I'd also expect a next-gen MacBook Pro to have multiple Thunderbolt ports for connecting a multitude of monitors, storage and pro peripherals.