@GtotheWaveDoes wizards still work with outside writers who want to publish D&D supplements? Or is everything in house now? we just had an open call for @DnD_AdvLeague writers – that’s the best place to start
— Mike Mearls (@mikemearls) March 26, 2015
Sage Advice: unfortunately this call is already ended in February 28th.
You can find some info here
maybe for the next call.