I cant help it.  My vision for the game changes every single day.  I'm trying to just lay the groundwork for what I know will be in the game.

I have terrain.  I have a player character that is animated.  I have damage, health, attack speed.  I can level up.  I have a bunch of monsters.  I have workers who can harvest resources.  Both workers and enemies can move around the map with pathfinding, taking tile movement costs in to consideration.  The player can harvest resources.  I have a working inventory.  Items can drop from enemies when you kill them.  I can consume potions to heal myself.  I have a tech tree and resource costs to build things.  There are factions.  There are towns.  There are roads between towns.  

I have no idea what my final game is going to look like.

Its an rts game.  No its an MMO.  No its an attack/defense/rpg game.  No its a wild west oregon trail exploration game.  No its a space game.  Actually I could...AGGHHHHHHHHHHHH

Actually I could... try to tell a story?

Does not need to be a complicated story. But even Oregon Trail has a tiny story. Some would call it a setup or scenario, but nonetheless, it is what elevates it above a puzzle like 2048. If you have rts elements, why are the factions fighting? Even birds vs pigs has an excuse for a story.

if your game its so big for you, break it to little things.. keep it simple then!

definetively not good that

you must having things clear if you try do something, and much times not simply "clear" also good balanced and calculated for it gives someclass of challenge to the players as Isr.goat said, keep it simple and with a objetive to reach

For example in my game Roguelike Heaven im doing (Is a 2D RPG game)

1) big maps conception pre-generation (cos its multiplayer then the idea of random-maps dont serve for this)

2) first of all a system combat of attack-strikesback with some class of weapons and basically with simple things as life bar, mana bar, thirsty bar, hungry bar and energy bar 

3) some other objects in map like for example places where you find some FOOD/Drink in map and also a system for get QUESTS/MISSIONS (this dont have to be too much complex, can be simply find an object somewhere in map/game and deliver to a certain character)

4) a system for interconnect the maps (i use a kind of DOORS/PORTALS placed in a certain zone of the map for jump from a map to another)

5) scenary complementary objets like TREES/Houses/traps hidden etc

Then later you can going over add more things and complexity with the modification of code

but one thing is obvious "Rome dont made in 1 day" and things is much better start in simple way and then retouching and fixing and adding new things to it cos with new versions updated and enhanced is how you going building a great game/app