From there, it is mainly raw content. The third phase will feature Alterac Valley and Warsong Gulch, while the fourth phase will add Arathi Basin. Blizzard does not change PvP with the fifth round of WoW Classic updates, but the sixth phase will bring global PvP goals into Eastern Plaguelands and Silithus.
There is just one problem: Blizzard has not set the timing for these phases. It's unclear how long it will take for a given phase before things happen. If nothing else, you will know how much you have to pay attention to your return classical arrives seriously.