Join Date: May 2008
Location: In space, targeting you with plasma cannons. (Actually I am from France)
Of the original Legions, I think the only ones that could be good again are the Alpha Legion and the Iron Warriors. The problem is that the first ones spent the last ten thousand years turning chapters and planets to Chaos, I doubt they would give up all their hard work, and the Iron Warriors hate the Emperor too much to serve him again, and they enjoy their freedom to much to let it go.
If you mean what legion could have been saved...well then pretty much all of them, maybe not the Word Bearers. A lot of little elements could have changed Horus's view on the Emperor and the Chaos Gods, first he could have avoided being wounded on Davin, he could also have ignored Erebus while in his "dreams", and so on... And Horus is the starting point, if he does not fall, then nearly nobody does: Mortarion and Angron are not convinced by Horus to join him to defeat the Emperor and serve the Chaos Gods, there is no threat to the Emperor so Magnus doesn't have to send a phsychic message using magic, Perturabo would have been able to take some rest between campaign and avoid the riots on Olympia, Night Haunter would not have seen visions of the Heresy and battles of Space Marines against Space Marines, and therefore would not turn insane.
And Fulgrim could have left the deamon sword where it was.