Lone wolfing it in creating games will have you face such times. You probably have ideas, just not ideas that translate well in classical gaming terms.
If you look at the example of the last game you made, a random Mario-like platformer, the idea seemed to have been playing around with classical formulas. Let's say we take super mario as a base, how can we build a game around it without it just being another mario clone? For example, let's say every mushroom makes you grow, but you cannot break bricks. At some point, mario is unable to proceed unless he gets hit by enemies and shrinks enough to pass through narrower corridors, which would be problematic if the player just killed all enemies, so he has to go back and find some. Or maybe avoid taking every mushroom he finds, though every mushroom taken is some more life (risk reward). Also, taking the fire flower makes you more powerful and able to attack everyone, but let's say the flame is also hurting mario if it bounces back on him(make the fireball last a little longer so it bounces around more). These are ways you can fiddle with a classic game idea/formula and make it more personnal.
Or you could take the other route, and look at the story of Mario. He rescued the princess and now is married to her. He is now no longer plumbing but king of the mushroom kingdom. He has to listen to citizens come with grievances and choose the appropriate response(accept,refuse, give item> example, a toad farmer is complaining about the goombas ruining his crops, you can send him off, be nice to him and lose the day spending time listening to his problems, give him a magic mushroom so he can grow and defend his crop more efficiently or give him the rare and expensive star and forget about this problem.). This would be a more kingdom management spin on the story with little to no reference to the classical gameplay.
Talking with people, having your ideas be challenged and brainstorming in a group is also a good way to force out some ideas you did not know you had. Don't give up and make something