Gamers have no shortage of ideas. Some of them are good ideas. Some of them are workable ideas. The intersection between good and workable is the stuff that might make it into a book some day.

The main difference between a professional (hah!) and a hobbyist I suspect is in evaluating the workable bit, not the good bit. 

One thing to be wary of is to get too carried away with an idea that sounds appealing, but which ultimately you will lose interest in. Not everything is inspiring enough to sustain you through a 20 page game, let alone a 100 page one. Are you going to try to build a hobby empire out of it like the Brutality guy, 2 Hour Wargames or Song of X&Y? It's gotta be an idea you can sustain interest in for months and years.

I use a notepad program to track ideas. If I think of something that could be fun as a project, it goes on the list. You can use anything, I use a little app called Tot which I use only for this purpose and nothing else. But you can just use a document on your computer desktop or a piece of paper and a magnet on the fridge. For me personally, keeping it separately helps compartmentalise a bit mentally. 

Every Sunday I sit down and look back over the list. I sometimes add a qualifier to an entry like "maybe" if I am starting to wonder if its a good idea or if I have changed my mind about some aspect (this SHOULD be a Renegade Scout expansion!). If I am no longer enthused, I delete it. 

It currently had 56 entries on it for consideration. They are not sorted in any kind of order.  The last 5 (you were gonna ask) are:

A skirmish game OR scenario booklet for "Snap haner" (Danish guerillas fighting the Swedes in the 1600s), some sort of Arthurian mythology game (probably a warband game?), an urban fantasy skirmish game with vampires and werewolves and all that, some sort of campaign game based around a space ark seeking a new world and a game or scenario booklet about partisan warfare in WW2 Norway.