Well, a number of things, according to the dev diary. The team is not looking for a "mystical silver bullet that magically makes everything better" but instead is looking at three primary areas to revamp. First, the team is taking a look at the many keeps, outposts, and the Delving. The current model rewards players for flipping control points to their faction's control, but there's no incentive to continue to hold them, so the studio aims to "shift the model to instead provide large benefits for having and keeping locations." Secondly, Turbine is analyzing Creep traits and skills for "usefulness, potency, and acquisition costs. Finally, all Creep classes will be receiving new skills and "an overall balance adjustment on... damage and healing" to ensure they are balanced with the Free Peoples.
The full dev blog is far too in-depth to post the whole thing here, but if you're interested in the nitty-gritty of all the PvMP changes coming with Riders of Rohan, head on over to the full post.