I'm planning on making an addon board for the Pi Zero that acts as a USB hub for it.
The idea was to have something with the same form factor that can be fitted underneath or on top of a pi zero that either has a micro USB cable to connect to the OTG connector, or just some wires that could be soldered onto the pads PP22 & PP23 (Similar to this hack: http://hackaday.com/2015/11/28/first-raspberry-pi-zero-hack-piggy-back-wifi/).
If it was a powered hub, it could even hook into the Pi Zero power connector to power both the hub and zero off the one adapter.
The only thing that's stumped me is that I've got no clue how to make a USB hub. Is the PiHub that Pimoroni released a few years ago open source? Or is there even a good open source USB Hub design out there that I could copy?