Prototyping and Validating your idea
As soon as you know your core mechanic/loop, it's time to confirm if it will work!
It’s critical that you prototype your core mechanic(s) as soon as possible.
The faster you have them working, the quicker you can establish what needs to be iterated upon, the more you iterate, the better your game will be.
However, don’t spend time on balancing values too precisely, focus on the broader picture.
Art wise, use default objects to get the point across for what you want to achieve, no need to integrate real art assets yet.
Sometime, you need to have an online component to your game, and that can take time to implement, doing an offline prototype on the side allows you to test your ideas and secure what needs to be replicated later. For your offline prototype, you can implement local multiplayer without much hassle, to try out specific multiplayer features.
Aim to have a working prototype as fast as possible.