Not sure if you've come up with something yet but I can give some things that help me come up with stuff.
For one, think of the games that you like and what you like about them. Then, try to see which of those things you want to (or can) emulate. And then from there, put a twist on it. Trying to come up with something out of the blue can be really tough so if you grab some things out of the air that you know you already like, it can make it a bit easier. That being said, it can still be tough to come up with a 'good' idea. But a good thing to keep in mind is a mediocre idea done really well is always better than a good idea done poorly, so if you come up with an idea that you like, but you feel you can do better it can be good to just get started and something really special might come to you later.