It would definately be exciting if Ash gained a different Pokemon, Pikachu was unexpected and cute back then, but imagine the possibilities if Ash somehow got... a random unexpectedly weak Pokemon like Pidgey or Magikarp (ish laughing).
I would have REALLY loved it if he somehow got one of those rarer Pokemon such as Sandshrew, Slowpoke, Growlithe or Abra. XP It would be very different experience for Ash!
In fact, it would give the anime a much needed edge and high drama. Think of all the fanfics you can write