I've used the SP, RP only idea this year in my Yahoo Roto league and it generally works out well. The only problem you run into are in eligibility issues i.e. Graves, Chacon, Shields (all relievers) qualify at SP and are then used as extra closers, when other teams can't possibly have the same amount of closers, but this does add strategy. At the very least, if someone does manage to find all relievers that qualify at SP, they won't be the best like KRod, Lidge, Mota, Gordon etc. I can't think of any better idea though and this in general safeguards against all relievers pretty well. Personally, I don't like the idea of taking out WHIP and I kind of doubt it would initiate a noticably greater amount of trading.