If you're gonna do a parody, i think you should get as close as possible to the real thing. Only if you change art styles i guess you get a little bit of leniency (like a 2d to 3d, or 3d to 2d etc)
Mainly story. And the biggest proof of that is that text-based VNs exist and are a success. Story is the most important thing, renders you can get better with time, but if you suck at writing and your ideas are trash, there's no way you can make a good game
Lose the spark that made the game unique. A lot of devs forget what made people like their games for the first time, and start people pleasing as soon as the money starts coming in
I think it's a little bit of a double edge. I want good devs to make money, but it always seems that as soons as the game starts booming, and the money starts flowing, the quality drops and the fan service starts