My answer is USB-C. Imagine for a minute that all of your devices (iPhone, iPad, MacBook, iMac) only use USB-C connectors for all peripherals. The advantage is not only the increased speed and power transfer abilities, but the idea that you only need ONE cable with only ONE connector. Whatever it is you need to plug in, and whichever end of the cable you're looking at, and whichever way the plug is facing (up or down)...none of it matters. It's one universal connector for power and data, with symmetry with respect to the cable itself and to the orientation of the plug and port.
Whatever kind of nonsense the people are talking about with the fragility of USB-C and/or the tendency to come out of the port more easily...that is irrelevant. The advantages are so dramatic. I can't even tell you how many times my parents (admittedly luddites) have tried inserting USB-C plugs into their iPhones by mistake, or brought the wrong cable with them on vacation. For the general public (who does NOT read MacRumors.com), life would be much easier with one universal standard for all purposes. Plus, it would be easier for people like us to not have to answer those questions about which cables are for what.
Does anyone else remember the days before USB-A? SCSI ports for printers, ADB ports for mice and keyboards, other random ports for connecting monitors to CPUs, etc., etc. Of course, the DOS/Windows computers had their own unique connectors which were not compatible with any of Mac connectors. USB-C is the holy grail they've been searching for! It allows you to connect everything to everything else with only one type of connector. How is this a bad thing? Yes, there will be a few years in there where hotels, restaurants, airports, airplanes, and all peripherals need to get on the USB-C bandwagon, but once the transition is complete, it will be SO MUCH BETTER! In the meantime, carry some USB-A to USB-C dongles, and you should be okay.