I have the 40 stm and the voigtlander 20. Both are great. Shooting manual focus at 20mm on full-frame is totally easy. The only thing is you can't shoot quickly one-handed but so what. The voigt shoots beautifully. Two lenses at 345 grams total. Share a 52mm filter. It's great. Amazing how effective the distance scale becomes as well as the focus confirmation. The 16-35 is twice the weight of both put together and over twice the size. It's a hassle switching lenses but the prime shooting experience is also special.
Having used Pentax for 3 years previously, and having owned a handful of their pancake and small limiteds, I'm most satisfied with the 6d and a couple of small lenses. A 24mm pancake would be very compelling. I guarantee it will be stm if it exists. I just can't see it happening though. I'd bank on another m lens with a new m camera that actually is worth buying. Canon isn't going to give up the mirrorless game that quickly. But if they don't spit out a modern 50mm they'll frustrate a lot of people.
i agree the voigt 20 and 40 are an awesome pair when travelling light
with the 20 set f8 hard stop infinity and everything from 2m to infinity is in focus anyway its a wicked point and shoot when used like this.