Personally, it seems to me like you want to make your own thing, but you don't quite know where to start.

If that's the case, the first step is to nail down what key elements you want to include. Giant robots, monsters, a city, whatever. After that, you'd wana work on filling stuff out by just asking "why" a lot and answering it.

"Why are there giant robots?" "Why do I need two people to pilot them?" "Why is this one city so important?"

If you work off that, things will start to fill out quite nicely. This even works well for specific characters as well.

"Why does the big bad evil guy wana destroy the world?" "Why does he hate the party so much?" "Why is the king taxing his people so much?"