you don't need incredible idea , but lot more some graphic style and gameplay :

- futurist corridor run with aliens to smash (you could run and attack : you could avoid or attack at the right tile in running in front of the alien

- Lot of traps : you must jump or avoir by strafing or do a special action (click somewhere on the screen etc ... )

- For themes :
Medieval as a knight
Japan
Western
on road using cars
Futurist in some planet caverns or outdoor paths
Underwater tunnels using submarine etc ...
jumping on roofs of houses
you have the choice

For graphics :
Cartoon
Wow like style
Borderlands style
Next Gen graphics (with cartoon propertions or realistic ones)
Simple retro textures and low poly (but well done) models
etc ...
Wireframe graphics
Flat polygon style (virtua racing like )

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Just an advice :
Make it as simple as possible , you can make perhaps only 3 tiles variations and use traps and other stuff.
Don't pursue the best game, make it and finish a level, it already be a bgi achievment

Even shorter :
No tiles variation, always same road/corridor
Only 2 style of obstacles
One character hero only (it could be an human,animal, alien etc ...)

Well anyone is welcome !
I hope even newbies to 3D will want to participate (it can be a good start to learn 3D game making) !