Many settings involving parallel worlds have an opposing group of some kind, from a more or less distant part of the multiverse, which I could probably describe as a difference from the baseline of Fantastic, Cosmological (from another planet), Biological (from alternate or future evolution on Earth) or Historical degree.
The heroes who oppose them might be agents of a protective multi-tangent power (probably making it more plausible that they'd actually succeed) with similar variations, of a secret organisation defending their own tangent (from its present, future or past), or of a national government.
They might instead be a capable group caught up somehow in an incursion, or college students. Or alien college students
with bad attitudes and unlicensed nuclear weapons. This post has been edited by uncle_jimbo on Apr 1 2018, 22:02