I think the cast should go back in time and run into the cast of a different series who have also gone back in time (maybe have Archer run into Sisko for example).

Kinda like the episode where DS9 went back to the TOS tribble episode, but this time have the two casts interact.

Maybe even have the crews have opposing goals. If either group achieves their goal they save lots of lives in their timeline, but a lot of the other crew will die.

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