I find it incredibly unlikely it will be 128GB. Apple hasn't increased the minimum since it introduced the 13" rMBP, so they're due for a bump. Meanwhile, the first gen retina MacBook, which was introduced after the last update to the 13" rMBP has 256GB. There's no reason for Apple to continue having the rMB have more minimum storage than the rMBP.
I agree, and hope so. With SSD prices continuing to drop at some point the cost of having SKUs for low end memory machines does not justify any additional profit made from larger machines. And 256 seems to be the bottom on many top tier models.
I believe the made the 15" models all 16 GB in 2014.