The concat_tables function currently requires schemas to be identical across all tables to be concat'ed together. However, tables occasionally are conforming on type where present, but a column will be absent.
In this case, allowing for null filling (or default filling) would be ideal.
I imagine this feature would be an optional parameter on the concat_tables function. Presumably the argument could be either a boolean in the case of blanket null filling, or a mapping type for default filling. If a user wanted to default fill some columns, but null fill others, they could use a None as the value (defaultdict would make it simple to provide a blanket null fill if only a few default value columns were desired).
If a mapping wasn't present, the function should probably raise an error.
The default behavior would be the current and thus the default value of the parameter should be False or None.