What about a super chain quest?
THE EVIL LEAGUE
1 - You find a secret soviet base in south pole, you get inside, and kill general Boris Skavuska, and you are able to produce UFOs. You find a map to a unknow insle near Iwo Jima.
2 - After you get into it, you find the mad nazi scientist Hans Frenauth. He was trying to spread the T-Virus, you kill him, you get the T-Virus, and you are able to make a T-Virus bomb with it, and also you can get 3 BFGs 9000
3 - You get into UK, and use the BFG 9000 to destroy a stone in the north of the isle. You will get Arthur Sword, after you use it, you summon the Knights of the Round Table. You get the map to Ierusalem, and you must get there to find the Holy Grail.
4 - After you get into Ierusalem, you meet Xerxes' army, You must fight and capture him. When he is near to die, he talks about a Nazi base in the forests of south america, and about a team of super vilains called THE EVIL LEAGUE, you will find the Holy Grail there. But before he can tell who is commanding it, a laser comes out from sky, and kills him. Then you must go into south america.
After the Evil League knows about you, they will start periodicaly sending task forces to destroy you.
5 - Using the map that Xerxes gave to you, you find a nazi base in amazon. You get inside it and fight against Nazi troops. You fight against General Hitler II, you kill him, get his Panzer XVI (a super super tank 3 times more powerfull than a T-95), he talks about Barack Obama and tells he is the leader of the Evil League: Osama Bin Laden, Hugo Chavez, Barack Obama, Kim Jhon Lin II, and Ghost of Sadam Hussein. Their base is in the North Pole. Yes, during the day, Obama rules the US, but during the Night, when everyone think he is slepping, he planning the destruction of the world.
6 - You get into the base, kill the guards, then kill the bosses, one by one till you kill Barack Obama. When you think everything is endend, Osama Bin Ladden shows up again, and he makes a fusion with Obama, turning into OBAMA BIN LADEN. After you kill him, he tells the truth about the Evil League: They were trying to call for an Alien invasion that would almost destroy the world. Only the strongest people would survive, these people would be captured by the aliens and geneticaly modified to create a super race of humans. They would use the Holy Grail to produce the new humans.
GRAND FINALE:
After you leave the base, the Covenant Aliens starts an invasion in Earth. All players get allied to fight against them. They send 6 Battlecruisers arround the world that will spawn fighters that attacks you. After you bring them down, the Mothership will come spawning more fighters. If you destroy it, there will be an ending cinematic like in Metal Slug 2, and every players wins the game by this secret finale. Everyone will get a password that spawns a Ion Cannon in Africa.