I would never go to production without a solid vision on the design. Lacking this will hurt the production and product so much...
You can see from a designer if this is not there. Answer to questions are not exact and game can change after questions or answer can be in contradiction with previous answers. Focus is lost and not solid.
Once the vision exists the game is solid and ready and most questions are answered.
Do the CORE EXT WISHLIST exercise!
You will get a lot of feedback from different people including producer, boss, team, friends and maybe even your sibling.
Take in the feedback and take what you think is best for the game and fits the vision.
Game designer is not the dictator stating what should be in the game, but a funnel for all the ideas that come up for the game.
For Tetris with the City building mode, our vision would be to "Create a great Tetris with as exiting city building mode as the basic game."
If our vision would be broken or missing, we'd say "We'll do a Tetris clone which has some metagame in it, which could have something to do with metagame building. We'll figure it out later..." Nope, please figure it out before production.