>>52sounds like you need some constraints,something to keep you within scope,This comes from either other people or self placed constraints.
When I say core features, I mean gameplay, not story or characters or lore.
Pick 5 gameplay mechanics/ideas that make up your game
Ex: sandbox focused on exploration, no set goals, point and click,party of 3, no quest failure
Now sit down and explain how you plan to implement that,what it looks like, how it works, and how it connects with the players options.
If you come up with some new "idea", ask yourself, "does it fit within any of these current features?" If so write down just the first line of the idea that you had, Ex: "timed quests would be cool", then go back to fleshing out and finishing the core features.
Once your 5 core features are done,then look at your list of other NON-CORE features you thought up along the way, pick 5 and repeat
if any idea doesn't work or you have to fundamentally alter your other main features, It just might not fit and you need to discard,dont go out of your way to cram something cool in.
Story,characters, and lore are all areas that you want your mind to go wild on. Not everything is going to be good, but you first want quantity over quality,then you go back and refine those ideas.
Its up to you to start with game play or world building first, game play is what you need to take it from an rp to an actual product and will be harder to design but is more critical if you want an actual game and not a novel
World building is where you add your flavor and will most likely come easier and in a rush, but its not necessarily going to get you any closer to having an actual game
world building needs game play, game play doesn't need world building.