Apple currently sells almost 20 million Macs each year, four times as many as a decade ago. So they must be doing something right. So I'm going to assume they'll want to build a new MacBook Pro that is a lot like the current ones, except that they want to start the transition to USB-C as that's the connector for Thunderbolt 3 and it nicely consolidates a whole lot of different stuff AND IT'S SMALLER!!!.
I'm going to assume the HDMI and SD card slots aren't going anywhere. They were added fairly recently and their usefulness has not declined since. (I haven't needed my mDP-to-VGA adapter yet this year as HDMI has finally pervaded meeting rooms, hallelujah!) I hear there are still a few people that use the 3.5 mm audio port, too. (Did you know that it also provides optical TOSlink digital audio out?)
So that leaves one MagSafe, two Thunderbolt/miniDisplayPort and two USB type A ports that may be replaced by USB-C ports.
As the owner of a now defunct PowerBook that I still have to figure out how to recycle properly, I can tell you that MagSafe is a very useful feature. I tripped over the PowerBook power cord, it went flying through the room, landed on a corner and I had to bend the corner back so DVDs would go into the drive again.
So going from MS to USB-C for power has a big, big downside. On the other hand, one power port to
rule power them all (iPhones, iPads, MacBooks) is extremely attractive, as would be the ability to plug in power on either side of the laptop.
An interesting option would be to keep the MagSafe charger, but provide a MS-to-USB-C adapter. This allows the user to plug power into any USB-C port but still enjoy the safety of mag. (On the MacBook [nothing] this adapter could also have a 3.5 mm audio jack so that wouldn't have to be in the computer and then you could have two USB-C ports.)
However, the problem with this approach is that you use up a valuable USB-C port for the mundane task of delivering power.
Verdict: keep MagSafe for now, but if possible, also allow charging through the USB-C ports so a new 5k display could be connected using a single cable. Also, users who feel strongly about charging the computer using a port on the other side get what they want and Apple gets to see how well USB-C charging works without having to take the blame if it leads to cable tripping issues.
As soon as Intel has Thunderbolt 3 chipsets, this is a no-brainer, as USB-C is the official connector for TB3. The question is: how hard is it to have more than two TB ports? The Mac Pro has six powered by three TB buses. So perhaps the MBP could have four TB ports powered by two buses. Or maybe that would require too much extra logic. Also, routing these insanely high bandwidth signals to the other side of the device could be problematic.
This could mean that the MBP remains limited to two TB ports. So then the old TB ports are gone. Not too big of a problem, as most Thunderbolt and DisplayPort devices use a separate cable which could be replaced without too much trouble. Notable exceptions are Apple's TB Gigabit Ethernet adapter, but that one can be replaced with a USB 3 one, and the Thunderbolt Display. I guess if Apple cares about backward compatibility (and they obviously don't as the TBD doesn't have any other inputs than TB so you can't connect anything to it other than a TB-equipped Mac) they can sell an adapter.
Oh right, there's also this quaint little protocol called USB that runs over the USB-C connector. If routing TB to more than two USB-C ports is not a problem, then a new MacBook Pro could be fitted with four USB-C ports, so you can still hook up a combination of two displays or TB devices as well as two USB devices.
But if there can only be two TB-equipped ports, then having four (or more?) USB-C ports would mean that there's two types of USB-C ports: the ones that can also power TB devices and displays, and the ones that can only do USB. That's not The Apple Way™: all ports must be fully functional. So this seems unlikely.
Verdict: we keep the existing USB type A USB 3 ports. Not only does that sidestep the same port / different capabilities issue, it also makes the transition easier because all your existing USB crap can be plugged into these ports and Apple doesn't have to start selling USB-C-to-lightning cables.
So we end up with a machine that has all the same ports as today, except that the Thunderbolt 2 / miniDisplayPort ports are replaced with USB-C ports, which will take power, run TB 3, DisplayPort and USB. Extra credit: migrate these ports to the other side of the computer by swapping them with the SD card slot so that it becomes possible to power the computer from either side. Or if routing the signals isn't an issue, have one USB-C port on each side of the computer.
Also note that now all the ports are (somewhat) smaller than USB-A so it should be possible to give the MBP a wedge shape without too much difficulty.