I can really only think of one fic that really treats Saruman seriously - my pet favourite epic series, which does huge wonders for complex plots, including portraying Saruman as rightfully persuasive and out for his own power (though it does have that he's been allied to Sauron, but even Sauron has been suspecting him of treachery, and sent an Easterling commander (one who has become something of a darkhorse favourite of the fic) to investigate and sabotage Saruman's efforts).