This is a thing bought up quite a lot.

I agree some horizontal progression could be quite good for the game. However horizontal progression has as many potential pitfalls as vertical progression and if done wrong it can make things really messy. I know as sure has hell there are things I don't want to see happen personally. If they are going to do something like this it would be something I would want them to take some time in and implement probably during an expansion.

I don't think any system used in another game will necessarily translate well into FF14 since it will have to gel with both the player base and games existing progression systems.

The other thing is that content like Coil and CT will fall out of favour anyway. Even if Coil and CT offered rewards usable at max level they wouldn't hold the popularity among the bulk of players compared to newer content simply because most players have already run Coil and CT a lot unless it was easier/more efficient in which case people would mindlessly spam it and complain about SE reusing old content.