Here are some ideas, even if not very original :
- a boxing game, like
Boxing on Atari 2600, or even better, in pseudo 3D like
Punch-Out!! on NES ;
- a naval combat game between two nineteenth century sailing ships, like the ones in
Broadsides and
Sid Meier's Pirates! ;
- a remake of
Tron Deadly Discs on Intellivision or Atari 2600, which had a fantastic yet simple gameplay ;
- a remake of
Shark! Shark! on Intellivision, another game with a fantastic gameplay. The player controled a small fish which had to
eat smaller fishes in order to grow and ascend the ocean food chain ;
- a burglary simulation, with a big part inspired by the Skyrim lock picking mini game ;
- a computer security system hacking simulation, inspired by the Mass Effect mini game.
If you are still looking for ideas, you may have a look at the fantastic
Hipopotam's digitalised collection of handheld electronic games on the excellent site
www.pica-pic.com ( Flash needed ).