This kind of cross-class freedom was literally one of the first things that went after Yoshi-P and his team took over 1.0, because it was an unbalanced mess and completely ignored established Final Fantasy iconography.

That said, I COULD imagine crazy amounts of cross-class skills being the schtick of Red Mage as it was always intended as a jack of all trades, master of none role.

(Except apparently in XI, where it was a DoT heavy DPS, which is half of the Arcanist's damaging moveset, so we likely won't even see that version in FFXIV.)