I won’t be building a e-MTB anytime soon, but inspired by @Frank 's planetary gearbox and carvon exo, this is something I have thought of. Hopefully someone will use/improve on this design. I haven’t design the gears because I don’t really know how to design the gears. Note that this gear only works with airless 6" tire (http://www.electric-skateboard.builders/t/diyeboard-com-china-factory-direct-retail-electric-skateboard-diy-kits-and-parts/32666/73) , as the air tube will be in the way of the gears. The hub I based on is @Maxid 's 6" hub cad.
(though I guess carvon might have thought of this already haha)
Connector to the motor ring (If you are wondering why there are 4 holes instead of 6 its because I’m working on a jacob hub build) The blue part is where the sun gear lay on
http://a360.co/2zoEg6C
Inside of the hub I assume using 10mm of meshing gears here, but I don’t know if it works or not. The outer design of the hub(where the existing star is) can be varied and should be covered completely to prevent dust and dirt.
http://a360.co/2D9FYuZ
An image of the direct drive, the connector and the hub
http://a360.co/2l6IGtC
Here are also some maths found mid-way: exo drive kv 90 optimum reduction ratio : around 1:3~1:4
P.S. Oh and If someone did use it, I would definitely want improved file 
(Im doing the same with air tire inspired by frank and my maytech hub too)