As you may know adding a new column to a MySQL table with millions of records is hard and in some cases impossible.
Is it a good idea to add a text column to each table (say, meta) to save some headache in future? Of course, they are empty in most cases.
These data can be stored as JSON. I know this field can not be accessible easily but it can avoid another join and it's important for this performance-sensitive app.
It uses InnoDB.