First, we have to assume how important xp is going to be. And basically xp is as important as the game makes it. Some games only use xp for lvling you character (low importance), some use it for lvling sub/professions and items also (medium importance), some games after a character is lvled use experiance as a currency (high importance).
So lets first speak on the main importance of XP, leveling you character.
Leveling your character once again has differant lvls of importance based on the game. From what we know about DD2 so far it should have a high importance. This is because gear has lvl restrictions. But once your max lvl how could xp be made important?
Well it can either be made into a currency, there are three ways this could be done. In all these below examples the experiance bar would stay perma-set to a large amount of xp, basically stay perma set to the xp required to reach max lvl from the lvl right below it. So from 69-70 in DD1.
1. When you reach max lvl the xp bar is still present, just now whenever you fill it again you will gain a XP coin/point to spend at a special vendor.
2. When you reach max lvl the xp bar is still present, just now whenever you fill it again you will gain bonus loot chest, this chest could have any random loot in it or some mana curency.
3. When you reach max lvl the xp bar is still present, just now whenever you fill it again you will start to fill reward tracks the lead to exclusive rewards, such as skins and pets that can only be earned by getting more xp in this reward track. This would reward players who put more time into the game than those who don't.
Any of these ideas would keep xp from becoming irrelevant, even in late game.
I really like the xp being a currency, maybe you can trade xp for mana in the tavern. The ratio could be 1:4