I highly doubt that Pikachu will ever evolve, and if it had to then it'll only be when the show finally comes to an end. However, like I read earlier, Pikachu was given two chances to evolve, and both times Pikachu itseld rejectef the option [The second time Team Rocket stealing the Thunder Stone]. Personally, I wouldn't want it to evolve either as it is basically the main mascot of the franchise and seeing Ash with one instead of Pikachu might come off as a little odd from the start.
Though, Pikachu stands for one thought: You don't need to be something else to be strong. You simply have to try your best and not give up, no matter how small or large you are and no matter how much bigger or stronger your opponent may be. Pikachu is like a symbol for the anime, and I wouldn't really want that to change.