This is How to Write A System Novel

In this introduction of the story, you want the reincarnation/transmigration/time-travel/encounter to believable and meaningful. Ask yourself: "What is the point of adding this? Does it add to the story in any way? Does it change the circumstances of the main character?".

Some stories will mix reincarnation with transmigration and needlessly add extra magical fantasy elements to the story while not making any of those changes useful. If the main character is reincarnating AND transmigrating, there better be a good reason for it. If there isn't, I'll assume that you don't care about your story.

Also, if the main character lived 1 million years, at least make their death believable. An example would be them trying to reach the highest cultivation stage and dying in the process or dying of old age while trying to reach immortality.

I also don't mind an enemy, but they must be overwhelmingly strong. If a main character is an old monster and dies from weaker students or some random weaker person catching them by surprise, you have nowhere to go from there. You're just redoing the story from scratch. They get to their old cultivation stage, and then what?

The whole point is for them to evolve beyond where they were in their previous life, not to just get back to it. Otherwise, where are you heading? Unless the point of the story isn't related to cultivation, in which case the backstory about them being a cultivation monster pointless.

Also, if they've just encountered an element of the upcoming system and aren't being transmigration/reincarnated/time-traveled, then at least that system should lead to advancing the story in a way that helps the main character grow in some way. If it's just levels, that's fine, but the encounter with the system should make sense. Randomly getting a system for no reason is a big plot-hole.

So now, they've been reincarnated/transmigrated/time-traveled or had an encounter and it's done for a meaningful reason that moves the plot forward and is a consistent part of the story. Now what?