Re: Time Travel and Potential Plots.
I ran a time-travel story arc a few years back where the characters ended up going into the future, seeing what the world was like and learning that they were the cause of it, and then having to come back to the present and try to keep it from happening.
Some of the details: One character survives an assassination attempt. The best the team can come up with for the shooter's identity is that it looks sort of like an older version—father, grandfather, whatever—of a gadget-wielding hero in another city. As they're trying to figure out what to do, the shooter shows up again and tries to kill the team martial artist (Black Cat). Bad idea. She cleans his clock and takes him to team HQ, where he reveals that he actually IS the aforementioned gadget-wielding hero...from 20 years in the future.
He explains that these two heroes were (or will be; the biggest problem with time travel adventures is knowing which verb tense to use) involved in a superfight where an innocent bystander gets killed. This bystander is actually the daughter of a mid-level Genocide leader, who uses the event as an excuse to start a holy war that ultimately kills the heroes.
This has an Archduke Ferdinand effect: a Genocide-hating former hero group comes out of hiding and declares war on Genocide. Superheores start choosing sides (destroy Genocide on one side, try to maintain the status quo on the other). Pretty soon the whole metahuman community is involved. Genocide, on the brink of losing the war, forms an alliance with VIPER for increased manpower and supplies.
With VIPER's resources and Genocide's technology, the balance shifts and the VIPER-Genocide alliance starts removing superpowered opposition at an accelerated rate. Within ten years VIPER has betrayed both Genocide and Doctor Destroyer and removed practically all opposition. End result: in the shooter's time (20 years in the future), VIPER runs everything, and only a handful of supers, a band of former heroes and villains now united in their struggle for survival, remains to oppose them. (And he explains all of this in exquisite detail in just a few seconds, thanks to the magic of the zero-phase soliloquy).
The shooter is recalled to his time by a built-in temporal homing beacon, and the heroes tag along. Once there, they see the conditions of the world, meet the Resistance (which includes a couple of notable enemies from their own time), and have to help fight off a VIPER death squad, in which fight one of the team's longtime nemeses is killed. Debating what to do, the decision is ultimately made to send the heroes back to their own time and encourage them not to let the innocent bystander get killed. And if she does, they'll have to find some other way to keep the incident from escalating into a war.
So, using various clues the shooter and his cohorts had given them, the heroes started trying to figure out how they were going to keep future history from repeating itself. Imagine the players' dread when the critical fight was over and they realized that they hadn't managed to save the bystander after all...