I think just making the license board much bigger, i.e. 3X it's current size and making you earn each HP and MP increase there (not automatically with level like now) would make it a lot more challenging and fun.
Limiting the number of active licenses like VeloZero suggested is a great idea too!
There was one thing that was more realistic on the Sphere grid: If you follow on the "healer" path you will have to start from scratch and take ages to get your healer to be a good swordsman. Makes sense. Real life is this way: Changing "professions" takes time. I wanted to be a singer, astronaut, ballerina and soccer champ when I was a kid, now I know that EACH path would take me a long time to get good at it so I have to choose one
The License board is designed this way but it is so short that makes it meaningless: any path way too easy to change and in 1 level your guy / gal is as good as the original healer or swordsman of the team. It would be much more "realistic" (how real can mages be ?) if License Board if it was much larger and trading paths midstream made you wait 10-15 levels to get good at the new path.