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 ).