And don't say anything related to a cookie clicker

Well, i have this handy little list here of all of the ideas i could think of for all skill levels
Beginner

Platformer, just a very simple level based platformer game, it can assist in learning how to make gravity based things and have objects interact with other objects, only really basic theme i can think of is like ninja related or something, though you can do what you want with it

Puzzle game, you could do a point and click style puzzle game, like an escape room or something, it would allow you to get better at having something happen when multiple conditions are set, you could do like a dungeon theme or lab style thing with this one

Intermediate
Tower defense, this is something that seems extremly simple, but it is kinda interesting and difficult when you actually try to make it, this can give you a better understanding of lists and custom blocks, you could theme it around a forest or laboratory

Bootleg AI evolution, this is a subject i have gone through a lot, its pretty cool and kinda interesting to watch, i named this bootleg because it doesn’t use the conventional method of having a neural network, you could see an example that i made if you want to check it out, i dont know much of a theme but i do cars a lot for this, if you want to challenge yourself you could do rag dolls

Expert i do not fall in this category, my ideas might be a little rough for this one
Physics engine, we know them, we love them, and they are very hard to code, this is generally a cool thing to make and since it always gets a lot of bugs it is really good for learning how to code stuff like this better, softbody physics are also pretty cool to do

A 3d game, this might be a little tough to make, but it is possible, you could do this

An Actual Neural Network Based Ai Evolution, yes, that is correct, you could try to make an ai evolution project with an actual neural network, i tried and failed, you can do it

Scroller platformer, this is something that might not seem that difficult, but its kinda hard to get stuff to move off screen without turbo warp, but this is just a platformer but the screen moves with the player and stays centered on the player so instead of you moving, the map itself is moving, dont really know any theme for this