Personally, I'd prefer to see the older animated shows on DVD than the sitcoms. Never really cared much for any of them,.. I just wanted them to be over so I could see Ren and Stimpy or something.
I know the original Ren and Stimpy has been released for older audiences, and Invader Zim has been released with blessing from Nick by a third party... but I'd love to see Doug (as long as Nick has the rights to the old series, I don't see the problem) CatDog, Rocko and Angry Beavers on DVD at least in some sort of compilation.
Plus, since the Spongebob season sets are doing well, why not have Fairly Oddparents in season sets?