I was thinking on this the last few days and I thought the best way to do this wasn't to totally remove the restrictions, but instead give players another thing to chase after.

Not a quest, but rather a reason to get more classes to 50! Get a class/job to 50, that class/job's outfits are unlocked for you.

So want to wear that nice looking BLM jacket on your MNK? Get a BLM to 50!