When we started to have an idea that this game was more story-driven than mobile skill game, we needed to first make lines within we stayed. It has by far, been one of our best decisions.
It's too clear for anyone that has ever made any kind of story or bigger project, that it's easy to get side-tracked and start focusing on creating more than editing ideas you already have. But, when we made lines and boundaries for the story and what the game was about beforehand we can now tell when we have an idea or a concept we can insert in the game or do we have a thing for world-building and possibly next game.
Couple of character example have larger now story lines, but we're leaving them to the background because we have made clear how long this game is going to be. (About 2-3 hours, if anyone is guessing.)
Our structure is going to be very classical 5 act structure. This how basically because our initial level design was made by @olenbluu, that is writer on the process, so we had that. 5 arc structure has been a good choice, though.
It gives a clear formula that you can use as a foundation and strict enough, but leaves room for couple of slower moments. Basically 5 act structure is just 3 act structure, but cut to more blocks, adding more blocks to BEGINNING, MIDDLE and END.
So, now it feels that best decision anyone can do before doing any kind of game development is to make a good plan for what your making. Then comes prioritizing, maybe you don't have time for everything. But even when this comes to you, you can go to your plan and see, what can you take away without ruining everything. And this is not only for game development, it fits a lot of other creative work. Plans are not the most media sexy thing to make, but they do help.
Make plans beforehand. It helps.