Captain Jigglypuff
*On Vacation. Go Away!*
I think it is a bad idea because viewers are so used to Ash being a protagonist for so long that taking him out of the anime would drastically change it for the bad. Pikachu is beyond iconic now and is the central face of the entire franchise. Remember how upset players were during Gen V that Pikachu could not be obtained in any way other than the Transfer Park and a couple of events? Cut it out of the anime would really upset fans. Most shows that change protagonists tend to fall in terms of quality and story telling. The only way that changing main protagonists could work is if it is done in a natural way. Goh could be a replacement but a lot of people dislike him as a protagonist and he would need to either get a Pikachu himself or some other Pokémon of equal recognizability and nearly as strong.