MySQL Schema Agility on SSDs by Tim Callaghan.
From the post:
TokuDB v6.5 adds the ability to expand certain column types without downtime. Users can now enlarge char, varchar, varbinary, and integer columns with no interruption to insert/update/delete statements on the altered table. Prior to this feature, enlarging one of these column types required a full table rebuild. InnoDB blocks all insert/update/delete operations to a table during column expansion as it rebuilds the table and all indexes.
Not sure how often you will need the ability to enlarge columns types without downtime but when you do, suspect it is mission critical.
Something to keep in mind while planning for uncertain data futures.