I think you have a point, but starters are usually more than "a mon from each type": Not only are they from specific elemental types, but their evolution line and power curve usually lets trainers use them for a long amount of time, longest than, for example, a Butterfree who could be left obsolete once you need actual power, without being a Noibat, Larvesta or Litwick, evolution lines who stay too weak for too many levels to make them useful on early game.
I think it could be cool to include other pokémon as starters, but it can't be just anyone.
(also you could always take debug mode or other mods to just give yourself the starter you want)