1. An asteroid, meteor, or comet is on a collison course with the Earth.
2. An intelligent super-virus begins taking over the minds of people exposed to it.
3. The heroes arch-enemies band together to take out their foes by trading opponents.
4. Atlantis invades the surface world because of pollution of the oceans and underwater weapons testing.
5. A super-villain attempts to trigger a nuclear war.
6. A vigilante is assassinating criminals in the city and the heroes must enforce the law.
7. Criminals have stolen advanced weapon prototypes they are using to commit robberies.
8. A “monster” threatening the city turns out to be a confused alien infant.
9. An omnipotent entity comes to Earth looking to have some fun “playing” with a group of heroes.
10. A fire rages through the city, threatening lives and property.
11. Mercenary villains are hired to capture or kill the heroes.
12. A tidal wave or tsunami threatens a costal area.
13. A villain transforms people in the city into creatures (demons, zombies, plants, etc.) under the villain's control.
14. A vampire is on the loose, feeding on innocent victims in the city at night.
15. A would-be sorcerer botches a spell, opening a gate to a demonic netherworld.
16. Cultists are kidnapping people to use a sacrifices to their evil god(s).
17. Villains commit crimes posing as the heroes to blacken their good names.
18. Shapeshifting aliens infiltrate the government and law enforcement and out law the heroes.
19. A villain attempts to use mass mind-control to take over a city, country, or the world.
20. A rampaging monster is let loose on the unsuspecting city.
21. An earthquake strikes a major city, causing numerous disasters and opportunities for looters.
22. Another team of heroes has gone missing.
23. Aliens invade the Earth. The “aliens” might also be from another dimension, demons, mutant monstes, etc.
24. A scientific accident creates giant insects that swarm through the city.
25. A villain captures the heroes in order to “play” against them in a life-or-death game.
26. The rightful ruler of an alien empire comes to Earth seeking help to reclaim her throne.
27. An alien warrior arrives looking for a worthy opponent to challenge him.
28. A villain attempts to steal all of the heroes powers for himself.
29. The heroes are kidnapped to become gladiators in an alien arena.
30. A mythic god starts a modern cult of worshippers, but isn't exactly what he seems.
31. An accident sends the heroes to a parallel Earth where good is evil and vice versa.
32. A villain attempts to melt the polar ice caps to flood the entire world.
33. A murderous spirit goes on a killing spree, possessing innocent host bodies.
34. A hero is infected with lycanthropy, slowly transforming into a were-creature.
35. A villain steals a powerful weapon and attempts to blackmail the government with it.
36. A secret and corrupt government black-ops organization tries to co-opt or eliminate the heroes.
37. A new crime lord begins consolidating power in the city.
38. Assassination attempts are made against government or public figures.
39. A villain steals the components necessary for a proceedure or ritual intended to vastly increase his powers.
40. A super-human with out of control powers steals the components necessary for a cure.
41. Villains abduct the heroes' loved ones in an attempt to coerice them or lure them into a trap.
42. The heroes are trapped in their own headquarters which is counting down to self-destruction.
43. The heroes are pulled into a dream world where their deepest fears and desires come to life.
44. A strange meteorite causes mutations and unusual phenomena.
45. An ancient alien “world killer” weapon is headed for Earth.
46. A powerful hero turns to evil for no apparent reason. Is he an imposter, possessed, or something else?
47. Alien bounty hunters come to Earth looking to collect a price on a hero's head.
48. A mythic figure challenges a hero to a contest of skill or ability to prove who is better.
49. The heroes need to find a particular rare device or item to save a friend's life.
50. The heroes discover a lost or hidden civilization isolated from the rest of the world.
51. Subterranean creatures “steal” buildings by pulling them down into the Earth.
52. New super-villains are actually aliens trying to get the supplies they need to repair their crashed starship.
53. The entire population of a small town has mysteriously vanished without a trace.
54. A young and powerful superhuman needs guidance in the responsibilities of great power.
55. A major new rock star is mentally influencing fans through hypnotic music.
56. A villain blackmails the heroes into assembling the scattered parts of a powerful device or key.
57. An innocent man on death row appeals to the heroes as his last hope.
58. Someone is sabotaging various high-tech companies without setting off any of their security.
59. Archeologists inadvertantly unleash a terrible force from the past during a dig.
60. A popular new nightclub in town is a front for criminal activity.
61. A psychotic killer leaves taunting clues for the heroes and the police.
62. Thieves hijack a shipment of valuables (gold, cash, jewels, etc.).
63. Terrorists seize control of a site with numerous hostages and begin issuing demands.
64. A ceremony to publicly honor the heroes is disrupted by super-villains.
65. A deceased hero (or villain!) leaves an unusual bequest to someone in his will.
66. On Halloween night the city is threatened by a plague of ghosts, zombies, or other monsters.
67. Someone conducts a media smear campaign against the heroes.
68. The heroes are invited to participate in a sporting event or competition for charity.
69. A hero's powers begin to go out of control.
70. Villains kidnap an important person for randsome, perhaps even a hero in his secret identity!
71. The heroes forget their super-heroic identities for some reason.
72. Everyone in the city suddenly develops super-powers while the super-powered people lose theirs.
73. The heroes are asked to help safeguard an important diplomatic meeting threatened by terrorists.
74. Super-villains hold up a bank or attempt a robbery.
75. A villain tries to steal all the parts of an ancient map to a hidden treasure.
76. An object on display in a museum turns out to be a powerful mystic artifact.
77. A scientific experiment causes time to behave strangely.
78. A hurricane, tornado, or other severe weather threatens.
79. Animals of all kinds suddenly turn against humanity.
80. Damage to a tanker threatens to spill oil or chemicals into the ocean.
81. A new designer drug on the streets gives addicts temporary super-powers.
82. Two warring alien races want to use Earth as a battlefield for their conflict.
83. A traveling circus or carnival is actually a cover for a criminal organization.
84. A villain places an indestructable force field around a city to turn it into a personal fiefdom.
85. An accident involving mutagens alters a hero's powers.
86. A villain brings cartoon, movie, or literary characters to life.
87. New threats are acutally the mental projections of a young mutant unable to control her powers.
88. Supervillains seize control of a small, isolated nation and the heroes are asked to oust them.
89. An accident splits the heroes into good and evil twins.
90. A villain poses as a hero to infiltrate the heroes' team.
91. A hero's actions or legal rights are called into question.
92. A villain kidnapps an NPC hero as the final component of a fiendish device.
93. A villain suddenly seems to turn over a new leaf. Is it for real or is the villain trying to fool people?
94. The heroes are shrunk down to miniature size right before they have to deal with another crisis.
95. Super eco-terrorists are threatening targets and the heroes have to defend them.
96. An omnipotent being pits the heroes against their arch-enemies to determine whether good or evil is superior.
97. A cosmic crisis threatens to destroy the entire fabric of reality.
98. The heroes are displaced in time and have to get back home without changing history.
99. A villain asks for the heroes' aid against another, more dangerous, villain.
100. A super-villain assembles an army of villains to conquer the Earth.
That should get you started. Oh, and "Time of Crisis" is a fine adventure, too.
