Personally, I would like to see every dungeon have the difficulty that AV did/does. When everyone is level 48 doing AV for the first time, that place is a nightmare, but fun at the same time because of the challenge. In order to bring this into new dungeons, my suggestion would be the following: Have each set of mobs you face before the boss get increasingly difficult. An example of such would be by allowing the mobs to place a de-buff on everyone in the group. Whether that be decreasing hp/mp/tp/str/dex/int/mnd that changes each time you enter the instance, so the de-buff you get is random. On top of the de-buff, have the group have a mini - enrage timer. Nothing uber of course, but maybe each mob gets a 5% damage increase for each other mob that dies .... or each mob of the group gets a 10% damage increase if they live longer than lets say .. 5 minutes. This would also make groups think twice before pulling 3-4 groups of mobs at once. A small change like this could see a rather significant change in the difficulty and add a new challenge to keep the instance from becoming dull.
*I am not a developer or know much about computer code* but these seem like a simple change to a filthy casual such as myself.