OK - it’s hard enough to come up with your own missions, so what do you do for the guys you don’t even care about? Give them the boring stuff. If 5,000 zombies are wandering the countryside and need to be put down - send your players against the necromancers and other NPC adventurers against the zombies. If a volcano erupts and shows the entrance to the dimension of fire, your players will not be the only ones to notice. Others will be racing them to acquire whatever treasures are in the plane of fire. Tapped for ideas? Use idea supplements like Spark of Imagination. No, that’s not a shameless plug - use whatever aids your want to. Remember - the other adventurers do not need to be as challenged mentally as your actual players. They also don’t need as much game balance. If it works for them to fight orcs and come away with a magical hoard of weapons - go for it!
The point here is two fold - the world cannot stand still while your players advance in levels. The world really does not revolve around them, and they shouldn’t be the most important people in the world, even if they did save it from destruction. The other side is the other side (meaning the other side of the conflict). If your players are good guys, then the bad guys will want to be gaining ground too. If your players are bad guys, then the good guys will want to rally the troops against them.
Maybe the gods want to get involved too. After all, if your players are slaughtering dragons across the planet in order to make things “safe” for humanity, shouldn’t the dragon gods get a little peeved? Wouldn’t they send their dragon champion on a quest to find the Heart of Dragonkind, a mystical amulet that allows the dragon champion to never be harmed in battle?
How does this play out? Well it could play out in a number of ways. The party could be back from killing the necromancer in a hidden tomb somewhere and learn that the two parties who were out chopping zombies to bits are the ones hailed as heroes, because they were the ones that the peasants saw protecting them. Or the party could spend three years saving village after village on the border, while another raided the tomb of a lich and returned with really powerful magic items. Or they could all just get along - comrades in arms, knowing everyone was doing their parts. When the final battle comes, your party might feel it is vitally necessary to protect the other party’s mage because he is the one wielding the wand of sunlight which destroys all undead. They might also see the evil high priest who sacrificed that one noble girl before they could save her and really want to get their revenge. Maybe we’re just making the point for permanent allies as well as the permanent enemies we advise in our books. Yeah - I guess we are.