personally..

I'd buff the exp from FATES, whilst also incorporating the gemstone system into it with various knick-knacks and mounts, expand upon the hall of the novice to create hall of the advanced, and attach appropriate rewards for incentive. Not gear-wise, but a minion/mount as away of bridging the gap a little bit, teaching players 'intermediate-advanced' mechanics. Further to this I would personally expand more upon guildhests, again as an additional measure to help 'mend' the disparity in ability between players.

I would have job quests also become mandatory beyond a certain point in the story. Level 40 MSQ seems appropriate for obtaining your job, at the very latest, and then maintain this by requiring the final quest of the base game require Level 50 job quest completion. Which is something I would also continue into HW.