Let's assume that we have some rules otherwise we could disregard the whole script. Time travel will be a must.
I think the biggest problems with Endgame were that a) it was essentially a remake of Timeless, and b) Janeway ended up being the biggest villain in Starfleet history because she destroyed a timeline for no reason other than her selfishness. She didn't like something, so she changed it.
I'd want to remove the Janeway is a villain factor. So I would give her a REASON to time travel.
So were Kim and Chakotay in "Timeless" (and Jake in "The Visitor") big villains?
The actual "Endgame" acknowledges that Future Janeway's intention is at best ethically dubious, and also has present Janeway (and, less believably, eventually also Future Janeway herself) reject it.