Enevhar Aldarion
Hero
Most of his magical training should well predate the branch off though. We have a pretty good idea where Loki was after that point and it added up to, first, sitting in prison on Asgard, second playing fake Odin, and then hanging out being a party boy on Sakaar. None of those were particularly likely to have provided much opportunity to learn new stuff (though its not impossible during the fake-Odin period).
I am more referring to training gotten at an earlier age. Variation in what was learned was not enough to trigger a pruning. After all, look at how old Sylvie was before the TVA decided it was time for her branch to go.