The original sin of pokemon was ever having more than 151 in the first place. Maybe you could add ~10 or so new ones with each title but the point is if they'd kept the number low they would've had so much more freedom to fan out in different directions and expand upon the world in ways that weren't strictly related to how many pokemon there were. More sophisticated capture mechanics, or a deeper battling system, or anything.

Of course this assumes that Game Freak were ever working in good faith and didn't just want to take the cheap and easy route of having 100+ new mons as a selling marque for the anime/plushie lines.