Linkin Park can only be improved by replacing all of the members with talented musicians and getting someone else to write their songs
An easy one is booting the shrieking pretty boy Dani Filth from COF (vocalists are probably the obvious selections in a thread like this). I might actually listen to them if they had someone decent on vocals.
On the same line, I'd like QOTSA a whole lot better if Homme would get John Garcia back on vocals (I don't dislike them though, so maybe they don't fit in this thread).
And Phish would have been a better Dead clone if they actually took drugs and Trey didn't sound like a pussy on vocals. They did their best to emulate over the years, but never did they create anything. That's a lot of wasted musical ability that one good acid trip might have corrected.