I know it's probably been discussed already, but an additional USB-C port would be so welcomed. Just simply for charging and connecting to a display (yes, I know it's not a pro machine).
I don't know if that means huge addtional costs for Apple to redesign internally. The outer shells with one USB-C would be useless or perhaps melted down and reused?
It's been discussed at length.
There are evidently support issues with offering two fully functional USB-C 3.1, much less TB3 ports with the current chipset, but I wouldn't expect design issues would be prohibitive. In fact, Apple probably designed it originally to have two USB-C ports, and changes made since appear to be preparing for that eventual update. I would expect such an update to go without much pre-leak fanfare given how quietly it could likely be accomplished.
The bigger problem is what to do about the removal of the 3.5mm jack. Apple does not currently support USB-C headphones, nor their own Lightning headphones for that matter. So they risk alienating a segment of their customer base by eliminating a widely used port, without providing any reasonable support to replace it. And in doing so, it doesn't really eliminate the need for customers who depend on physical ports from carrying a USB-Hub, but requires them to carry yet another dongle just to plug in their headphones. Perhaps that could be a strategy to move headphone users to W1 chipped BT headphones, but Mac users tend to use wired headphones for higher quality sound, and lag free applications. Then again, that same strategy is currently driving the single-port experience, where Apple wants you to connect to the outside world wirelessly, via AirPlay, AirDrop, and WiFi.
You're looking for a second port for charging, to keep the one USB-C port free, which is why I thought Apple might add a Lightning port -- meant exclusively for charging and audio, but could be used as an optional USB 3 port with adapters Apple already makes and supports. However, it doesn't appear that Apple intends to widely support Lightning-based audio across their products, preferring to push wireless audio, which fits with their stated intent on the MB from the start. So my next thought is that Apple will replace the headphone jack with something like a Smart Connector which brings back MagSafe in a fashion for charging, as well as offer a wired audio port for those who may still need it, once the iPhone has moved more customers to wireless headphones. Indeed, the smart connector is already in use on the iPad, and could be planned for the iPhone once they introduce wireless charging, and remove the Lightning port. So there would be a unified charging and wired audio/USB 2 solution across all products.