Are there any plans to add extra backgrounds in the gamemaster's handbook? I would personally love at least another 18 leaving us with 36 total. Or go extreme like Into the Odd and have 60! The more options there are, the less of a chance of having repeat characters.

Extra backgrounds is not something on the plans for the GM's Handbook. That's something more inline to other future materials, and we also feel like we covered all the bases with those 18. 

Also, don't worry about repeating backgrounds! In my experience those proved to create more interesting and diversified characters than the opposite.