Concerning Besieged: I think there are two types of changes that could be made: Obvious and Unlikely, though I guess SE going back to change something like Besieged makes it all unlikely.
Obvious: Adding drop pools to the Beastmen Leaders(and making their bonus 100% in Besieged while they remain defeated). Nerfing Instant-Death/Doom on NPCs and Nerfing some Beastmen Bosses(and some 2hrs). SE has shown the ability to change the method of Campaign's exp/an gain with ease, they should be able to do the same to Besieged's exp/is gain... increasing the maximum reward for 60+ and lowering for 30-. Change "Besieged:" into "Assault:"(and let them work during Besieged, instanced Campaign Ops, and Campaign Battle). Finally, Auto-booting "Away" or unactive(for 20 minutes?) players to Aht Urhgan Whitegate.
Unlikely: Extend Besieged to Wajaom and Bhaflau. Have Beastmen attack the semi-useless Watchtowers and allow players to slow down Beastmen on their way to the capital(and once they do reach the capital, players are awarded and instant warped into Al Zahbi). Players under Besieged would be unable to attack or be attacked by non-Besieged monsters. When under the effects of "Besieged" 12% movement speed(same type a Sprint, stacking with other gear) and the ability to spend 25-50 IS to be warped to a watchtower. Add quests/tasks which allow powerhouse NPCs to gain in on the fight like the ones we see in quests(Mishhar and his crew[lulz]/Ghatsad) or missions(Raillefal/Karababa/Naji). Give a way to weaken Beastmen so they can be taken down by a party of 6 with not too much effort, probably related to the first thing.
Now, Campaign is pretty well built, but it doesn't really uses all of its stuff.
Personally, I think the only problem with NPCs are that sometimes there are just too many. There should never be more than one group of soldiers deployed at once. Whatever is making them do that, have it instead increase the stats or the number of that one group. One more thing about NPCs, though I don't know if it's much of a problem, is that fact they should be able to grab some hate. Even if it's 1/3 of the rate of players, some jobs perform much better with a (temporary) tank(s). I guess something else needs to be said to: Let them stay around Fortifications. Or if they must charge forward, have the Allied Tag NPC make /says about where the battle is currently located. Another thing they could do is only have inactive General or Freelances(with green names) standing around while the area is not participating in Campaign Battle and have them summon their soldiers only when Campaign Battle begins.
Beastmen who fail to capture an area grow stronger in numbers/general strength/rate of strategy increase in only that area(well, not the strategy increase). The growth is similar to Besieged's(increase exp/an caps?), making an area loss almost inevitable over 2 months without a lot of focus on Strategy.
For variety, it would be nice if some areas(especially in Fort Karugo-Narugo's many rooms) had battles in more than one location. Have the Allied Tag NPC warp players to the location(and a NPC who is always there to warp them back) for free and have Generals/Freelancers instant warp to the location which is being attacked. They could even go one step further and add variety to the objectives, like finding/protecting coffers to gain/lose extra funds or planting a bomb(which could have the same effect as the Sabotage Op).