Can you elaborate on the last point? Do you mean things like macros they use, e.g., concepts lite, or __builtin* functions?

I still feel like a standard container is less fundamental than keywords, but I value your perspective.

Yes, built-ins, intrinsics, and extensions.

My original point was simply that std::vector, say, is just as good a "keyword" as any - perhaps, not because you cannot redefine it but because you wouldn't, and if you didn't like what it does, you'd use/implement something else.