The most ridiculous idea is Apple adding a second port to the rMB 12, it's not going to happen. The rMB is more of a statement than a product. Sort of like: "This is our vision of the future of the notebook." I think the one port decision conveyed that pretty clearly.
Edit: to clear things up, don't expect to see more than 2 USB-C TB3 ports on the next rMBP redesign. Those controllers only support a max of two port, and split the bandwidth when used simultaneously. Unless Apple decides to use 2 separate TB3 controllers the possibility of a 15" rMBP with 3 USB-C ports like das fuhrer suggested above, is next to none.
I guess back in 2008 - 2009 when Apple introduced the MBA someone here said exactly that and two years later: boom, a second usb port and a sd-card reader.
One thing is aim to the future, the other being dumb. One port, the same port you use for charging is dumb.
Fun fact: people is actually making money with usb-c adapters for the MB12 in kickstarter. Look at this, looks like a
prosthesis to give it back its usefull limbs.