Depends whether by "in theory" you mean, "hey, man, anything's possible" or "with enough > effort, this could happen". If the former, well, yes, any article could be an FA --- in the same > way that a Muslim could become the next president of the United States of America. If the > latter, then, no, not all articles could become FAs. That's one of the reasons why GA was set up, and one of the reasons why GA is such a failure.
The rules for FAC specifically say that FA status can't be refused for a reason that can't be fixed, so by the letter of the law, all articles can become featured with enough work.