Potential for redemption?
With 40k Endtimes looming closer than ever, I pose the question: Is there any of the traitor legions you think whom might rejoin the Imperium, given sufficient motivation? The endtimes for Fantasy had shown that the forces of Order can have strange bedfellows.
The Alpha Legion is probably the most obvious choice, given their noncommital attitude towards chaos, as well the policies of the twin primarchs. Though it might have taken a hit after the death of Alpharius.
My other pick would be the Thousand Sons, sure Magnus have done some bad things after the heresy. But he still seems coherent, and I think there could be potential for him earning the forgiveness of the Emperor for his collosal mistake in breaking the Imperial Webway. Magnus only reluctantly joined chaos to save his sons.
Out of the remaining legions, I think maybe only the Iron Warriors might accept an imperial pardon. Looking at the sensible Warsmith Kalkator for example. Though in general their long term festering bitterness might be too much to overcome.
The Black Legion is a definite no for obvious reasons, and likewise the Word Bearers. The Emperors Children and the Death Guard are both far too corrupted, and the World Eaters are nutso berserkers lost in the red mist of Khorne. Finally the Night Lords have a long standing grudge with the imperium.