Anyone who uses the EOS M with the EF adapter probably knows the major problem with this theoretically great setup - balance. EF compatibility was the number one reason I bought the M, mainly for lighter overall weight and no shutter slap. This works ok with prime lenses on AF, but with zoom lenses or manual focusing, it really makes balancing the camera difficult while turning zoom or focus rings. The grip on the M is way too small to support the weight of bigger EF lenses, not to mention the EF-M mount wasn't meant to support much weight, hence the tripod mount on the EF adapter.
So I got to thinking, why not an accessory grip that screws into the bottom of the EF adapter? It would be very simple to mount a bracket on the tripod adapter (or even better, remove the tripod adapter and screw directly into the EF adapter, with a rounded joint for more support) and shoot a molded grip out to meet the existing grip. It would just need a half inch to make a really nice solid place to hold on to.
I'm throwing the idea out there to see if anyone else likes it, and perhaps someone with a 3d printer would like to try prototyping one. If there's enough interest, maybe it's worth a kickstarter campaign.