me personally, I would stay away from open worlds as a solo developer or a even as a small team. There is alot to manage, lots of swapping in and out, whether its memory, AI, collision etc. youll be needing a lot of tricks up your sleeve to handle things.

There are so many questions to ask yourself as early as possible:

how will you handle AI thats out of the player's range? Despawn, Freeze etc. Youll need to know from the beginning before you code AI. Will you abstract the AI when its out of range (doesnt play animations or anything, but teleports to its target based on an average time)

How will you stream your world in? multiple models loading in on distance? Then your level will need to be designed with this in mind from the beginning, cant just design the whole world in one model file

how will you handle memory loading and unloading? How will you load that detailed landmark model without noticing frame stutter? Can you correctly unload that landmark when youre far away?

And this is all framework stuff, it hasnt even touched content. You cant just replicate the same trees, you need multiple kinds of trees, for different areas, multiple enemies/pedestrians. Cant have the same track suit jogger pedestrian in a snowy area.

I think its better to have a smaller pseudo open world/area and really work on making it efficient.