I've been recently thinking about this topic too. Sadly, I have yet to arrive at a (fully) satisfactory answer. Here are my thoughts:
First, the end goal is immortality, right? so I'd start with thinking about what that looks like practically. My thoughts on the matter would be that means an immortal cannot "die" in any way or be trapped/sealed (which would effectively be the same as death). So my thought was that an immortal would be able to live with only his soul, and from only his soul, recreate a physical body. The soul would be untrappable, and even if his physical body died (say, he wanted to fake his own death, so allowed himself to be killed or something), he could recreate it immediately, so he can't actually die.
To prevent the world being filled with immortals, my idea is that they would all leave earth and go to some immortal realm which would serve as an equivalent of heaven. The immortals would then only interfere rarely if they needed to, but most of the time would be willing to just let nature take it's course.
So now I have my endgoal, I ask myself how a mortal could reach that endgoal. With that, my thoughts would be that a cultivator would slowly learn to detach himself from the physical world (literally, rather than emotionally), and learn to "walk" as a soul. The endgoal being that he can sustain himself infinitely in this soul-realm or something.
With that in mind, my thoughts were to split the cultivation stages up into three groups.
The first few cultivation stages would be mortal stages, which focus on refining the body for a long life, and for it to be healthy and capable of the next stages.
The second set of stages would be semi-mortal stages, where you start moving your soul around and learn to "walk". The idea is that these cultivation stages wouldn't affect the body, but it would give you more control over your soul. Furthermore, because these stages affect the soul, this progress would be partially kept upon death. (Hence they are semi-mortal, as they still die, but they carry over some abilities onto their next life, so they aren't fully subject to death.)
My thoughts are that, at the end of these stages, the cultivators would need to undergo this "ascension" thing. Here, they convert their bodies into pure energy to try to swim to the immortal realm. If they succeed, they become immortals. If they don't, back to the cycle of reincarnation they go!
Final stage would be immortality, which would be the endstage. You reach here? Congratulations. Thats it. No higher stage to reach than immortality. Enjoy your paradise in the immortal realm with everyone else who reached this stage.
And thats about as far as I got in terms of a power dynamic for a cultivation story. I also had this idea where "spirit roots" were sort of like the answers in a personality quizz. So if you were a fire-type spirit root, that just meant that you were over-emotional and short tempered. That way, different "sprit roots" would learn different techniques that suit their style.
