The first option (no explicit syntax) doesn't address my primary concern. It means that there's no way for a type checker to distinguish between "I forgot to add (or haven't gotten around to adding) type arguments" and "I explicitly and intentionally mean `Tensor[Any, ...]`.
The second option requires us to expand the syntax and increase the complexity of this PEP. I don't think we should do that either.
I understand your point that `[T, ...]` may not make sense for some variadic generic types (except the case where T is `Any`). I guess that doesn't concern me as much as it concerns you. It simply won't be used in ways that don't make sense. I also don't worry so much about users getting confused because they're already familiar with the semantics of "..." when used with tuples. But I understand your points here.
As a compromise, how about if we allow the syntax Tensor[T, ...] only if T is `Any`? In other words, `Tensor[np.float64, ...]` would be flagged as an error but `Tensor[Any, ...]` would be accepted.