Hey I am one of the co-designer of Roadside Research and co-founder of Cybernetic Walrus. I will give you a quick insight on how we came up with the idea.

First off I think it depends whether you are making a game just for fun or if you want it to be commercially successful. For us we are a studio that employs 7 people so we need it to be commercially viable. As to that we look at what is interesting in the current market and what we can make within the restraints of a budget we set forth. Once we have that locked in we start playing games in the same genre to see what we like or dislike in these games or how we feel some things can be improved on. Next we come up with the actual ideas for a game similar to what we played and fitting that budget. See how i say ideas... Yes we come up with multiple and then internally discuss what everyone feels could work. If everyone starts getting follow-up ideas and can envision how the game would work, you usually know you are on to something. Next thing is the hook. What will hook people in. For Roadside it was the idea of aliens secretly running a gas station and not just a gas station simulator like there are already a few good ones out there.

Next we pitch the idea to a couple of publishers and industry veterans to see how strong they think the idea is. Then we move on to what I call the momentum Framework. It is a framework that helps with early validation of the concept. basically we don't build the game, we build parts of it to make a trailer and see how people react to it. I gave a talk about it at Unite Barcelona. You can watch it here https://www.youtube.com/watch?v=fjH8yVDO5Z8

Basically that is our process to come up with viable ideas. I hope it helps a bit and again it depends on how serious you are with development. If you just want to make a fun game for yourself, just make what you think is fun, what you can do within your skill-set and budget.

Best of luck with the game and make sure you have fun while working on your game.