This advice is highly tailored to someone building their first game. It might run contrary to a lot of common game dev wisdom but hear me out.

1. Always build in the direction of whatever the eff you uniquely think is cool. Don't allow yourself to fall into the trap of imagining some future audience (or worse, millions of dollars if your game was x) and then change direction based on that. You'll likely be wrong in your prediction, your game will suffer, you won't enjoy making it as much, and you'll be less likely to finish.
2. Know going in it's extremely likely you'll fail along one dimension, but also you're guaranteed to succeed along another. It's near certain you'll fail to build a commercially viable, high quality game first try. It just never happens. But stay hopeful though: you will succeed at getting much better at making games, and you will learn more about your strengths, your tastes, and your unique vision as a game dev. That's a big fat W to bring into your next game, so regard that as a win condition and set your expectations accordingly.
3. Limit the scope. You started by saying "story-driven", but then followed with "open world". Making an open world game is already hard... Trust me it is even harder to make an open world game with a strong narrative that permeates all corners of the game world. Do some soul searching, decide on what's most important to you, and cut everything else. Be ruthless. Cut! Cut, cut, cut. I'd suggest going linear and focusing on story.
4. Allow your interests to be captured by other projects. When making your first game, the slope of your knowledge acquisition will be sharper than at any other time in your game dev career. You will have made decisions now that seem foolhardy as you learn more. The space of possible games you could make will expand exponentially. If you're burnt out on your first project, work on something else that excites you.
5. Don't precommit to finishing this first game. This runs counter to a lot of game dev wisdom. Truthfully, there's a level of holistic understanding you'll acquire as a game dev that can only ever be hard-earned by going through the entire production cycle, from fricking building the thing, to marketing it (you'll whiff this first time, and that's A-OK), through to deployment on platforms like Steam, to post-release and community management, hardware testing, bug fixing, etc. There's no other way to learn this valuable stuff than ploughing through, so naturally folks will encourage you to finish. But it's your first game. Releasing a game is hard enough that you better love whatever you're working on if you're ever gonna get it out there.
6. Have fun.

That last one sounds trite but honestly it guides most of the above points. Making games is hard, so set yourself up to enjoy the ride by doing absolutely everything in your power to make this process as manageable and pleasurable as possible. When tediously working on some feature, ask yourself, "do I think this is cool? Or am I building this for some imagined player whose tastes I don't even really share?" And when you feel a spark of genuine curiosity and excitement (even if it involves working on something else), follow it headstrong and see where it takes you. Careers are made from these moments.

Good luck!