I wanted to use see what people could think up regarding if certain characters reached their "full potential."
Some notable ones:
Darth Sidious wanted to subject every being in the galaxy to his will, quite literally creating a hive-mind, where he channelled his consciousness through every sentient being. He would go from galaxy to galaxy doing this until he became "omnipotent."
Anakin Skywalker, as we know, would have become the equivalent of The Father, who represented the Force in balance, and was the most powerful Force user. The God of the Force.
Darth Plagueis achieved biological immortality, and also learned how to increase his own midichlorian count. He also was able to induce "pregnancies" in animals, that is asexual reproduction induced through midichlorian manipulation. Not to mention, he found an alternate pathway to Force powers one may struggle to attain naturally, and it "led from a place where the circle closed on itself and sheer will substituted for selflessness. Plagueis understood, too, that there were no powers beyond his reach; none he couldn’t master through an effort of will." He did this by spending 20 sleepless years in attempt "to wrest a few last secrets from the Force." Therefore, in Plagueis' case, he mastered virtually every Force power and had mastery over life, death and immortality in the most fundamental sense; sans Anakin, he could have been the single greatest mortal Force user ever, bar none, had he been given enough time to increase his proficiency with midichlorians.
The implications of this are that he would essentially see the world and it's living inhabitants as building blocks of life, which he could manipulate and alter as he see's fit. Who knows what this could allow him to do in the long-term; mastery over life, death, time and space.
Darth Nihilus had he been allowed to, would have done quite the opposite of what was described above; he would have been the death of the Force itself, a black hole sucking everything in.
Valkorion would have consumed the entire galaxy, and perhaps others.
Darth Tenebrous had plans to create a retro-virus which would be capable of severing the Jedi from the Force, although that never quite panned out. He also had a plan to upload his consciousness into midichlorians and infect the Chosen One upon his birth, allowing him to become the most powerful Force user of all time. Although whether or not that could have really worked, if he could have truly achieved Father-level power by migrating into Anakin's body, who knows.
Any other characters who had serious unrealised potential?
__________________ “The galaxy must experience the pain of death and the rapture of rebirth as I have. I will bring chaos. It is time for war.”
Registered: Jan 2018
Location: In between parallel universes
Jerec was pretty close to becoming quasi-omnipotent near the Valley of the Jedi. Jacen became comepletely one with the Force in his battle with Onimi, Mother Talzin when she almost used that Ritual on Bardotta to become as powerful as TCW Sidious, and pretty much all of the Celestials, since they could all become Ones.
Although, it IS unclear how that works, since the Father, supposedly born 999,000 years before the golden age of the Celestials, was far more powerful than the Son and the Daughter, who were born even less powerful before the whole pool of knowledge and font of power incident. And the Father was the only celestial (as far as we know) to ever reach Force Wielder level power. And Abeloth was even less than a celestial, but then became more powerful than the Son and the Daughter combined after using both the font and the pool.