Having spent far too much time playing with Medium already there's one tool I keep wishing I had - something to sharpen up edges. It's especially needed after a resolution increase. My workaround is to use hard-edged stamps but it's tough to get the exact profile you want and can get messy. Flatten isn't doing it for me because a lot of the time I don't want to remove as much material as I end up losing.
Sculptris has a pretty awesome crease tool (https://youtu.be/GRiXO7E6zbU). Would love to see a similar implementation in Medium.
I'd also love to see a pinch/crease tool. In the meantime, I'm using the pointy primitive stamp for creases, and for edges I dollop on a line of extra gloop then carve it out with a custom stamp. The pinch needs smoothed after and the crease is a two step process. Not ideal but it's enough to keep me going. Hopefully santa will leave a pinch/crease tool in my stocking.
(sorry about image quality but I'm at work just now so I'm stuck with MS Paint)
Another vote for a pinch/crease tool. Any maybe an option to create a spline, and use it as a path to follow, rather than just the straight line modifier we have now. Being able to have a stamp follow an adjustable curve would be amazing.