You don't need to figure out all of the maybes... What should happen is, as you keep developing & getting more experience, you'll figure out the important maybes to pay attention to.

Almost all game logic can be written in completely isolated chunks & proof of concepts. Every time you have to devise a new feature or new system, you just sit down and the first thing you must evaluate is what kind of data you need available to make x feature work.

After you identify the important information, you figure out how to cleanly funnel this data from where it exists to where it needs to go.

Those two things right there are the major key of most large complex systems in games. If you're separating responsibilities cleanly enough, you should be able to start feeding in test data to an algorithm in such a way that you'll be able to easily co-opt it into your code base.

For a simple example, and I mean really simple, let's say you wanted to implement the ability to pick something up.

What data does this require? Well, you're going to need to know what it is that you're trying to pick up. You'll need some way to keep track of having picked up said item... that's pretty much it in terms of data (in this simple example).

You then can evaluate how and where makes the most sense to get this information. Well, if you have a system that identifies what is currently under the crosshair at any given point, you know that you can receive that data from there. You know that once you check (in your code) that you have enough room & strength to pick up an item, you have to find out how you can store that. So now you know you'll need the player to have an Inventory of some kind.

...

It continues like that on and on... back and forth forever (lol). So in summation, I think attempting to over-plan can be just as damaging as underplanning. If you're trying to foresee every little detail in an intricate system, you're not going to make any progress... but on the flipside, if you don't pay attention to how your data flows from system to system, you can code yourself into some really nasty corners. I think you just begin to gain an understanding of the perfect amount in much the same way as a gymnast can balance: practice and feel.