It is a bad solution IMO. Let's say you want to play GNB you have to level WAR or PLD which outside of your liking.. but you are forced to play with it.. It will drastically reduce player enjoyment.
Current dungeons easy and forgiving enough to figure out what to do as tank or healer.. easy to learn from your mistakes, most players in game patient and friendly if you tell them you are new at your job or dungeon.. all you have to do test a few skills on dummy and read skill tooltips, arrange your hot bar accordingly for your priority and adjust hotbar in dungeon.
Edit: +1 Undeadfire's suggestion, trust dungeons easiest way to figure out.