Hi Ida!
Some things I would love to see come to CSS in 2017 are subgrid and 3D CSS transforms on child SVG elements.
Everything I've heard from Jen Simmons and Rachel Andrew lately has gotten me super excited about grid and what it's going to do for layout on the web. And it sounds like it will be more powerful and fleixble with subgrid too. (If it's not already on the roadmap for support.)
Not being able to use 3D CSS transforms on elements within an SVG gets me every time. I keep forgetting that it's not possible. I should probably ask for more SVG attributes to be accessible via CSS first, but this one is top of mind right now.
I'm also excited to see where the Motion Path Module goes in 2017.