One of the biggest issues players currently have with city siege is that it features way too much PvE and not enough PvP (or in WAR's case, RvR). Patch 1.3.1 is introducing several improvements in this area. Sieged cities now have three states: Contested, captured, and rebuilding. Attackers and defenders will be present and battling it out in each state and the contested state public quests will benefit from a new design. Warlord encounters are now instanced and balanced for 24 players, allowing multiple groups to participate. Finally, there will be a number of siege UI improvements allowing you to keep better track of your realm's progress.
Nearly every area of the game is being touched in some way by the 1.31 patch. Take a look at the comprehensive patch notes on the Warhammer Herald to observe their true breadth and feel free to give them a try on the public test server.