I think we don't actually need a way to add more active players into war, because we are still suffering from inactive people in the real fighting, so why add more people if we already have the most active people (at least, that is what it should be) and even those aren't enough to fill a whole war.
Also, as Higs said, with rewards too high, the side events will be able to decide the war on their own, but if the rewards are too low, it won't keep people motivated, which leads to even more dropouts.