ROADMAP: add consistent missing values for all dtypes to the roadmap
The more detailed motivation is described in https://hackmd.io/@jorisvandenbossche/Sk0wMeAmB, and many aspects have already been discussed in https://github.com/pandas-dev/pandas/issues/28095 and linked issues.  
(and there are probably still other details / practical aspects that can be further discussed in dedicated issues)

Last year when I made the `pd.NA` proposal (and which resulted in using that for the nullable integer, boolean and string dtypes), which described it as "can be used consistently across all data types", the implicit / aspirational end goal of this proposal for me always was to *actually* have this for all dtypes (and as the default, at some point). 
I tried to discuss this goal more explicitly on the mailing list earlier this year (in the thread about pandas 2.0: https://mail.python.org/pipermail/pandas-dev/2020-February/001180.html). But we never really "officially" adopted this as a goal / roadmap item, or discussed about doing that. 
Proposing to add a section about it to the roadmap is an attempt to do this (as that is actually how it's described to do it in our [roadmap](https://pandas.pydata.org/docs/dev/development/roadmap.html#roadmap-evolution)).

The aforementioned mailing list thread mostly resulted in a discussion about how to integrate the new semantics in the datetime-like dtypes (pd.NaT vs pd.NA, keep pd.NaT but change its behaviour, etc). This is still a technical discussion we further need to resolve, but note that I kept the text on that in the PR somewhat vague on purpose for this reason: *"..: all data types should support missing values and with the same behaviour"*

And the general disclaimer that is also in our roadmap: *An item being on the roadmap does not mean that it will* necessarily *happen, even with unlimited funding. During the implementation period we may discover issues preventing the adoption of the feature.*

cc @pandas-dev/pandas-core @pandas-dev/pandas-triage 