I'm fascinated to see how nullable reference types play out, in terms of adoption. That goes for a lot of other things - there's a cadence of adoption, when libraries, tutorials etc start to make it important to know about features. C# 7 tuples don't have much usage yet as far as I've seen, but could become really important over time. Who can say? I'll be watching closely...
I'm interested by the shapes proposals, but I'm not sure whether it might be a bridge too far. (That said, I've usually been wrong about things like that.)
"Extension everything" is interesting, and I'm expecting to have fun using it to come up with some really evil code.
More generally, I'm looking forward to seeing how the C# team thinks about not bringing new features out. How can the language stay relevant without becoming a kitchen sink? Which features will be retained only for backward-compatibility, but not used by new code?