These would be a good way to alleviate the campaign grind and get people to play new or underpopulated campaigns. What I propose is an event where you complete challenges to get an XP boost.
For example:
An event which lasts 7 days. Each day there are 3 new challenges, and for each challenge completed you get a 10% XP boost in all campaigns that lasts for the remainder of the 5 days. This stacks with other challenges, so if you complete 1 challenge you get +10% XP, if you complete 2 you get +20% XP, and so on. By the end of the 7 days players could be getting up to 210% more XP towards their campaigns, but of course this would be limited-time.