Honour, Hope, Love
I'm definitely late to the party. Since the staff is unwilling to penalise players who do not participate in a war, the only real way to increase war participation is to give certain incentives to take part in the war. Another issue that AQ in particular has with wars, is the issue of how rewards (EXP and Gold) interact with time (Real life time.)
As many before me have pointed out, it is simply less efficient to farm a War as compared to a preferred Farming spot. This is due to an innate design within the game. AQ is a prized resource that is necessary to improve our character. This is incredibly different from other games, where "Item hunting" is the main way of obtaining better equipment. As such, in AQ "time=money". If one wants to increase participation in Wars, there needs to literally have a "time=money" incentive.
This is not helped by the recent slew of tank monsters that have appeared in Wars. The one that stands out most is the EbilCorp Medic (Lower level Hackers.) it's far from unbeatable, but the amount of time it takes to defeat him is outright ridiculous. It's far more efficient to fight even the EbilCorp Hacker in the Void based off the HP to EXP ratio.
This is further compounded by the stat system. Most Monsters above level 120 have points pushed into END, making them even more Tank than usual. This has been partially solved by pushing points into other stats. I'm not too sure if it was a ninja-hotfix, but at the later part of the last EbilCorp war, I found myself fighting a lot less Medics than the first few days. With less of the annoying tanks, I found myself racking up kills a great deal faster. Having less Tanks in a war pool definitely would increase participation, but it doesn't solve the root problem, that a War-zone just isn't efficient.
Now, I'm not saying to convert all the monsters in the War-zone into Monster Power 4 and give them a cannon lean. I'm not even suggesting to remove Tanks entirely, there needs to be some diversity, but we have to be realistic, AQ is a farming game, and there needs to be a kind of resource incentive. Monsters in a War-zone need to be competitive with "Farmable" monsters. As such, I would suggest a 1.3 multiplier to all rewards given in a War-zone.
If the carrot fails, there's always the stick. The last time an old fart (who's already participated in over seven years of Wars) like me was pressured into participating in a War was in the Shadow War, when Lord Barrius basically threatened us into action. EbilCorp wasn't much of a threat, since realistically, erasing characters would never happen.
Carrot and stick, hopefully we'll get the carrot... But the stick is a great incentive as well.