Now that I've made a 3D cube, I can make simple 3D games. The only problem is that I have no ideas about what I should make. Any suggestions?

You could try something simple like a Ping Pong Minigame.

First you should work on implementing backface culling, only draw the front 1-3 faces, and ignore the faces not facing the camera (Algorithm in screenshot, it is the same as checking if the Z cross product is >0 or <0

Nat are you gonna post a key?

you should now make a simple 4D hypercube

AlgorithmZ- That's a great idea, but I feel that it wouldn't be nessary for the game.

I took the idea from Gale Storm and I'm now working on 3D pong.

I know this would be hard, but if after ping long what do you think of a block builder? (I know probably not) Also I'll be looking forward to ping long!

Probably not, only because rendering more than 20 lines causes the game to lag. So rendering multiple twelve-lined cubes on the screen would simply be to laggy.

Ok :) That's ok