How do we come up with Game Ideas? Well, first of all, you have to know why you started this road in the first place, for Ideas, you just use what inspired you to make a game to begin with, even if the idea is a carbon-copy of say any 'Final Fantasy' game or anything, there's you're idea, then you think what you want to add, what other things inspire you, history can help along with a mix of life own life experience can set you up for a decent story line. Mechanics, I'm not good or familiar with it, but if you're starting as I think, you should just go with the basic and 'grow up' from there, look at other people's projects, what have they done, get ideas, explore the possibilities and don't ask how, instead just do it, Great things have the smallest beginnings, So, aim high but don't put yourself with too much work, work with what you have, explore and learn and add it, do what you feel is right, because little by little, you're game will take the shape of that story you want to show us.
I hope that helps you.