I don't see the field size ever changing - too many logistical issues with that kind of move (they're not paying for stadium remodels if they're not paying for grass fields), and that would be a much more foundational change that a typical rule change, and people don't like foundational change. More likely to me would be rule changes like adopting the college rule where offensive players can block downfield before the pass if it's caught behind the line of scrimmage. Give the offense a chance to attack more downhill. Or perhaps a return to more downhill, power-based run schemes as opposed to zone schemes configured within spread formations. Or more radically, an adoption of some A11-type principles that would use a defense's aggressiveness against them. Or in the opposite direction fun-wise, getting even more punitive on the defense in terms of who is deemed a defenseless player
Who knows. It's fun to think about though. I just don't think they've exhausted all options scheme-wise and rule-wise where they'd have to resort to something as drastic as widening the field