What kind of payment model would you like to see? Personally I like the idea of having the box be free but still charging a subscription. You could offer people a free 10 days or so with unlimited content. After that they pay their 15 a month.
Edit: below is something i added after starting this thread which i think is important to express.
Which is why I think removing subs is a bad idea personally. I do however think that removing the box fee from at the very least everything but the current expansion is a good step to go. Then have a massive marketing campaign about how you dont need to buy wow and you can play free and experience everything for like 10 day to a month. Send out emails to everyone that ever quit and invite them back for a week or two. Also for the people who feel that the immersion has left the game, they should give a reason to not teleport somewhere for the good stuff. Dungeon finder should exist, and should give valor. If you form a group and travel there however, you should get even more valor and more rewards. Half the reason no one travels to dungeons anymore is because its more rewarding, and more efficient to just queue. Those are a few ideas i have to set wow back on track.