Started a Homebrew (first DM ever) and I could REALLY use some help!

So, I started my own idea for a campaign, and I the players seemed to really like the idea.  We're a time/dimension traveling agency hopping back and forth to save important heroes because cataclysm is imminent and reality is coming apart with adventurers and heroes having become so scarce.  The idea is, we're a group trying to find the right way to intervene in situations where there was a total party wipe and stop that from happening so they can continue in their adventures. 

I have some imagination, but not as much as I'd like.  Do you guys have any suggestions or past experiences on fun, crazy, or harsh experiences in campaigns where things took a bad turn?  And if possible if you could include some vague details on what classes, levels, enemies we're involved and the setting it would be greatly appreciated... and the motive for the party to have been there.  Thanks.

Have the players actions in the past come back to screw them over in the future.  For example, save someone that starts an organization that starts out good and eventually turn to evil.

And/or do the reverse - have a friend they meet later originally be an enemy and their actions turned him friendly.

Have a blatant paradox that is unexplained.  Wait a second, how can we kill George when we know he is alive later?????

Have some people be really mad at the party in the future, but they do not know why until they go back in time and screw over that guy.