This concern is understandable. I just finished Sinisistar 1 and 2 back to back and this is exactly the same pitfall the dev's talking about. Sinisistar 2 is bigger and meatier than the first game, but if you look at the bigger picture, the progression is exactly the same, just more polished and more bells and whistles. Even the ending CAN be the same with both final bosses appearing as the same Eldritch slugs, which makes sense since both games are following the rules set on by Bloodborne on how otherworldly being enter their world.
That said, I said pitfall, but by the time I finished both games, I simply wanted more. There's nothing wrong with sticking to the formula IF you know how to make it better than it already is. Dude needs to believe in himself.