I have 2 ideas.
1:
Like you stated, a horror game.
You will be walking in corridors, and the map will be pretty huge but its all corridors, rooms and tunnels, and its free for all so you walk alone.
And maybe add some cool weapons like traps and other stuff to improve the gameplay.
2:
An island multiplayer shooter game.
2 teams on each their island, its boats and helicopters which you can enter and its deathmatch or capture the flag. Also a good choice of weapons so it becomes a little rock paper scissors, like rpg or at4 takes vehicles, m4 takes infantry etc.
