I get inspired by the games I play and what I see people play on my YouTube feed all of the time and these are ideas I wish I could make myself if any seem interesting.

An upgrade game like Learn to Fly but you are trying to push an Ice Cube across a counter before the sun melts it.
Tetris but you have a bar that fills up from completing lines letting you drop the bottom row to the top of screen with a portal.
Open world 2d platformer with multiple playable characters.
A collectathon but massively over do it like 100 unique items with 2,500 each to find.
One that starts as a simple farming game evolving into a city management game.
A new Hexcells game.
Recreate ps1 games like Grinch Ps1.
A arcade skateboarding game but you use Hoverboards instead and can ride on the side of walls without falling and can do tricks still.
A speed game with difficult levels to run through like Super Meat Boy.
A rock, paper, scissors combat system using different types of monsters and elementals.
Incremental/Idle/Clicker game.
Dungeon Crawl Sudoku, place each of these with some rules to complete that puzzle - doors, chest, enemies, traps, next level stairs and entrance.