Please allow for state in extension everything. I do this already with System.Runtime.CompilerServices.ConditionalWeakTable<T> but it's really ugly, add some syntactic sugar here or at least tell me why you are punting on this. Is there something I'm missing, this is like a mixin at that point, right?
I'm a big fan of Functional enhancements but I would put all them aside for the performance enhancements you speak of. Much less copying, pointer returns, Native Buffers, all that sounds great. I even wrote my own buffer wrapper around VirtualAlloc so I could get aligned memory that worked super fast with file system pinvokes. Was working on a super fast .net persistent queue and write ahead log. To be honest I would love to see tons more performance related enhancements to the language.