Separate NaT values for Timedelta ("NaTD") and Period?
Separate scalar missing values Timedelta, Timestamp and Period scalars would go a long ways towards achieving predictable types with pandas. As noted in https://github.com/pandas-dev/pandas/issues/19124, it is impossible to make some operations consistent with the current state of affairs. Most recently this came up in https://github.com/pandas-dev/pandas/issues/24957.

This is listed in the pandas2 tracker (https://github.com/pandas-dev/pandas2/issues/74), but I think it might even be achievable for pandas 1.x? There would only be backwards compatibility issues if people are explicitly checking object identity against the `pd.NaT` scalar, which is a bit of an anti-pattern.