One of the benefits of Apple including USB-C on the new MacBook is that it's a standard connector. Since USB-C is also how the new MacBook will charge, that means any battery extender made for USB — as long as it's within the proper power range — will work just fine.
The same holds true for external monitors. USB-C supports DisplayPort 1.2, which means — once the right cables are available — you should be able to output up to 4K video.
Since video adapters will likely sport additional USB and power inputs, or perhaps some future display will include those things built-in the way Apple's current Thunderbolt display does, those who want to plug in at home or at work should have a lot of options.
All we have to do is wait for everything to ship.