Here's a little example:
Before the mission has been cleared, it's darker on the achievements list. (As they are before you clear them in 2.7.) There could be something like "(500XP)" written underneath, to show what you get for clearing it.
Once you've cleared it for the first time, you are awarded experience for the wolf you were using at the time, and you unlock it as an achievement, which then gains colour.
You can then clear the same mission, just with another wolf, to gain the experience for that one. (Or you can clear it again with any wolf, but if you clear it again with the same wolf, the experience is reduced... Or something like that.)
It's just an idea, but I think it would give people more incentive to collect achievements, as it benefits their wolf/wolves. It may result in certain achievements having to be removed, but I think it would provide a better use for achievements as a whole.
Edit: This post is now outdated:
loboLoco wrote:We are designing something new to replace XP, but you will be rewarded if you’ve earned a lot of XP in 2.7. We plan to archive all 2.7 achievements and have a new, larger set of achievements for WQ3.
I thought this post might still have a use as inspiration for new ideas though, just wanted to mention that some parts would not work with WolfQuest 3.