I think it would be interesting... not if Pikachu evolved earlier, but what if Ash let him evolve now? Raichu's are known to be a bit more aggressive, so what if after Pikachu evolved, he'd become aggressive? Making him a bit harder for Ash to control, leading to a slight character development as he tried to tame the powerhouse that is now his Raichu and Raichu could easily walk beside him rather than being carried.
Actually no, Raichu could ride on his head like Hippowdown [sp?], even though that thing weighs like a ton, Ash still managed to carry it like it was Pikachu, laws of physics don't apply in the Pokemon world, so Raichu could easily ride on Ash's head. Team Rocket would just have to try ... umm harder? Oh who am I kidding, Poor TR Trio is nothing but comic relief and electric fodder for Ash and Co, sadly, cause they could do so much more.. Jessie would make a great coordinator and James could explore the world or something, with Mime Jr and Meowth tagging along... since I have no clue what Meowth could do with his life .