Think most of it comes down to just having a good idea, doesn't need to be complicated or even have fancy graphic. But it need to catch peoples interest fast by standing out. For instant if you take a top down space shooter, I think it will be very difficult to make a popular one, if you just copy what is already there, but if you changed it so maybe the player could build there own craft, collect materials on the maps or something that makes it stand out. Its might have a chance.
But also choosing a popular genre and execute it better than what is there will help, for instant looking at a game like Kingdom rush 1 and 2 which is very popular and good games. But breaking it down, its actually a pretty simple tower defence game, that could fairly easy be made in C2, but they have executed it very well compared to a lot of the others out there, think most know that there are load of those types of games out there, which are not very fun for whatever reason.
Also I don't think, thinking to much about how much money or success a game could have is not very helpful. But instead focus on making a game that is actually fun instead, and that you enjoy playing or think is missing out there. And as already suggested by other, make a website for it, post it on varies forums when you feel its ready for that, and then if its a good game people will start show interest in it, I think you will find it much easier to make some kind of success out of it that way. At least that's my plan....now I just need my game to actually work the way I want it to