They hinted to her becoming more but it just never happened. During the time she was under Shadow Kings control it was clear that her powers were boosted by his own telepathy she had a limit to how many people she could possesse back then, she wasn't shown the ability to take over a lot of people until X-force had the battle with Selene over the runestaff, and even with Shadow Kings help her powers were useless against Illyana and Warlock (who only needed to shield according to Illyana), why she was able to erase Illyanas memories in that New Mutant limited series was just as stupid and the limited series itself, since even Xavier Emma Frost and Empath could never breech her psi shields, she had only ever been affected by a telepath when Emma directly attacked her astral form while she was looking for kitty.