This is a great idea, I'll definitely be sure to implement something like this!

I think one of the best ways to be 'creative' on command is to impose restrictions on yourself, and just push to make something within those restrictions - whether the output ends up being good or bad. Doing this generally gives me a point from which I can either scrap the idea and try again, or iterate on it until it's good! I find that most mechanics tend to be quite versatile, and sometimes it is just a case of taking the idea to it's logical conclusion. Also, consider how X will interact with each other element of the game - this can help make even the simplest mechanics feel alive and varied because they then inherently have versatility. Got a grappling hook platformer? Make the grapple hook affect enemies to pull them closer. What if the grapple hook could be used on objects in the environment? Perhaps it could be used to prevent an object from moving, because it's tethered to something else. How would the mechanic be different in a different genre? A grapple hook in a 2D platformer would be totally different to an FPS, and completely different again to a grapple hook in a racing game. Being creative on demand is really difficult and it's something I can struggle with sometimes too. All we can do when we feel 'creatively exhausted' is to look for inspiration elsewhere, whether that's games or even real life. How have other games progressed something to make it better? Can we isolate the 'fun' part, and then take it further? Hopefully some of these ideas help aha. There's definitely a skill and process to master in there somewhere, and I think it's something we are all trying to work out!