A bicycle with an extended wheel base, like the 1) cargo bikes 2) bikes where the battery goes behind the seat tube 3) cruiser bicycles that are stretched out. I tried to do a mid mount dd hub in the triangle of a Norco cruiser, it was a half baked plan.
I liked the idea of bypassing the direct drives friction by using my Cyclone freewheel crank, which is a big bonus.
If I were thinking of doing it again, I'd look at these kinds of frames.
This picture below would be extreme, I would have 275mm space between the seat tube and the rear rubber. A large dd is 230mm, you can see the diameters at https://www.ebikes.ca/tools/spoke-calc.html?hub=MX_45&pair=false
Then you'd have plenty of room for a battery inside the triangle.
This one is unique, I kinda dig it.
https://www.electricbike.com/wp-content/uploads/2012/06/P10000801.jpg
Another unique design
https://farm8.staticflickr.com/7197/7098559047_3563e5d316_c.jpg
http://endless-sphere.com/forums/viewtopic.php?f=6&t=38553&start=25
What'd be cool is a custom frame job, where the crank, pedals and hub motor are all one piece, then it would look like a funky crank to a casual observer. Well kinda like this one, 20. Binova Flow - https://www.electricbike.com/mid-drive-kits/ thats the look from the side to any casual observer, obviously a hub would be a lot chunkier.