I've seen lot fo professionnal games doing iterations
Graphics was every time improved.

In fact they prototyped quickly a plyabale game with in progress graphics.

Another thing we could insits a lot is :
2D concept art and 2D level concept.

It's the main essence of a game , if you don't have precise idea of the models you want , textures, architecture and levels, style of game , precise
gameplay etc ...
you won't go very far

I think no need to modelise, texture etc ... before a fisrt big shot
of 2D art (documentation/storyline/game mechanics/progression).