BUG: Fixed concat with reindex and extension types
Closes https://github.com/pandas-dev/pandas/issues/27692
Closes https://github.com/pandas-dev/pandas/issues/33027

I have a larger cleanup planned, but this fixes the linked issues for now.
> I have a larger cleanup planned

Can you give a brief overview?  ive got a couple of branches going, may need to mothball them until you're done > Can you give a brief overview?

Nothing actually written down yet, and no timeline. Just a general sense of "this is probably more complex than it needs to be". So I wouldn't recommend planning around me. Might conflict/overlap with #33486 needs rebase just pushed a rebase Just (force) pushed a rebase.

I think this will fail till https://github.com/pandas-dev/pandas/pull/34338 is in. @TomAugspurger this will need an update now https://github.com/pandas-dev/pandas/pull/34339 is merged Merged. No need for `ignore_2d_ea` now that #34339 is in. This should be good to go now. thanks @TomAugspurger 