Grease pencil
please implement this feature
vpaint is open source and can do automatic inbetweening:
http://www.vpaint.org/
Perhaps blender devs could take a look at the code there for grease pencil tweening? 
The data structure of the grease pencil would have to be changed for this i think…
Thanks for that link, blurry, I hadn’t seen that one before. It’s even distributed under an MIT license (i.e. Blender could potentially use code from it!).
The video in the OP is also quite interesting. Near the end, an example of a stick-figured walk cycle is drawn (with application’s assistance). Imagine this linked to a rig, each of the “lines & balls” attached to joints in a rigged model. The animator sketches the action, with auto-complete, and the connection to the rig animates the character in 3D.
As Yoda would say, “Potential there is”
@raziel_henzo: Yep, I saw this video too yesterday. I’m definitely interested in looking into something like this for Grease Pencil. There are also some other alternatives I’m interested in playing around with in this area. In any case, I’m looking forward to attending their talk at Siggraph Asia, which IIRC should be held the day after my talk about Grease Pencil 
@BTolputt: It’s probably not that well known (at least in these parts) but I am in fact currently working on a lot of stuff along these lines. There’s a lot of interesting stuff along these lines to explore
Hearing that Aligorith is looking into this makes me excited about the grease pencil’s future!
Btw also check this paper out:
https://graphics.ethz.ch/publications/papers/paperWhi10.php
Perhaps blender could become the best open source 2d animation application as well as being the best open source 3d one?