While I do feel for those who need to level at a slower pace, I don't think those people are high enough in numbers for this to be put as a high priority on the list of things to add to the game. I would put it pretty low and tell those who want it that it likely wouldn't be added to the game before you wouldn't need it (ie: you'll likely be at max level before it gets added).

There are already ways in game to slow down the leveling pace and/or the addition of other stuff.

1. Level an alternate battle class. When your main class gets too high for your liking, switch to the other class and do things on it until it gets too high and switch either to another one or back to your main.

2. Hold off on the MSQ until you've had a chance to process things that have been added. As the MSQ is usually the driving force between the addition of new quests, dungeons, and/or trials, holding off on the main quest would let you get a feel for what was added with the last one before moving on and potentially adding new ones.

3. Making sure you're not wearing anything that gives increased experience as those additions typically give a decent boost to exp gain.