This idea has been thrown around for a while. Cool idea, but it'd have to be done in such a way that it wouldn't undermine the current skill leveling process.
Would it cost actual money?
Would it just take an awful long time to grind out? Ie: Guild Points out the whazoo with tiny success rates
Would it be limited to a time frame? Ie: 2 weeks on, 2 months off style level rotation
Would it affect current metagame too much and force Guild Wars to increase difficulty again, thus hurting the new/introductory player base further?
Lots of questions with very difficult answers.