Do simple stuff. Tic Tac Toe. Then make it 2 player. Connect 4. Then make it 2 player. Mess around with blackjack, then make it 2 player. etc... Build up your expertise.
Also, https://forum.unity.com/threads/game-mechanic-ideas.801474/ https://forum.unity.com/threads/wha...e-of-game-to-make-with-unity-and-why.1135249/
If you're interested in action games you cant go wrong with an infinite runner like Jetpack Joyride, Flappy Bird, or Subway Surfers. Just give it your own spin. Don't just make a clone.
Just lurking the forums after I've been gone for a while. Did not expect to see a thread of mine to show up on the first thread I checked :0 I agree, not making a clone can keep your motivation up as whatever spin you decide to put on a game will be something you are personally interested in. Aside from that stuff how about something like pinball? Perhaps a good exercise in basic Unity physics
Remaking Frogger is a suggestion I often see. It is pretty simple to implement in Unity, and is still a fun game. You can make your own spin on the game as well. For example, my favorite Frogger knock off of all time is "Senseless Violence 1: Survival of the Fetus", where you play as a baby crawling your way across instead of a frog, and die in shocking ways for the time (Apple IIgs game).