Wait... a new game dev seeking advice without a huge scope, unrealistically complex idea for their first real game?! 
Joking aside, I like to start my game designs with an idea; usually funny to me, and from there, the game play or core mechanic that I want to feature. An example would be if I wanted to have a game about giant monsters destroying a city and say I pair that with a rhythm game mechanic with a single touch interface. Anything past that is scope creep to me and is treated as such.
It helps to keep the scope from ballooning out too much as well as staying closer to the theme and you don't dilute your game play by sticking to your core mechanic.
But from what it sounds like you're on the right path so my only other advice is to come up with an idea you can tolerate from concept to complete game. If you are just making something to make something, you'll get burnt out fast. 
Hope I was some help and good luck!