Revamped Tribes

As long as we're going back and revamping the tribal quests and the ARR content to streamline the flow and add more drama to certain moments...
I think it'd be good for ARR's combat-job tribes to be brought in line with the rest. Having the 4 of them use 40-50 Level Scaling rather than being flat levels would be a notable improvement to the feel of the quest lines, and potentially a boost to people's xp to get them into the more recent fare.
The Ectahl Nine probably ought to stay as it is, considering its scaling style is so different from the rest, running all the way from 1 to 50.

ARR quests would need some modification to the quests themselves. Unlike most (not all) level-sync'd quests in HW an onwards, the quest content exists outside the normal state of the zone. A common quest asks you to go to location X,Y and fight an enemy sync'd to your level that spawns claimed to you, or pick up some bauble from the ground. Neither of those are content other players can see. ARR quests mostly have you attacking "in-world" enemies. The quests were based off of sending a level 48-50 adventurer through a certain area killing level 48ish enemies.

Some of the other quests are also just out of line with all other tribe quests since then. ARR quests tend to have many steps, journeying to more areas in the zone for just 1 quest.

They'd need to rewrite a decent amount of it. However, the content was doable in ARR and still doable today with the benefit of being able to fly in the zone if you meet certain requirements. The key problem is the exp reward simply doesn't match the effort required. Level syncing would help with that.