Can I make a suggestion?

I had been thinking of this AMAZING idea called "Dolly." It is based on a Dream world this 6-year-old is in. Her parents are both radiologists and one night she wakes up and hears a loud noise. She wakes up and goes to her parents room. Their bed is empty but the closet i wide open and glowing with this huge light. She goes in and comes to realize that her parents are not who she thought they were.
Inside there was this huge lab and a chemical explosion killed both of her parents. Now, what they were experimenting on was a doll that the main character called "Dolly" (hence the name of the game) Basically what they did was put a heart inside the doll and it becomes corrupted and starts to give off these chemical fumes. And its alive to not forget to mention. Now, the little girl runs and the doll chases her. She goes back to her room and locks the door. And she goes back to sleep.
That is the end of the monolouge. Later that night, she wakes up again. And that is where you would start playing. There would be a key on her nightstand and she locked herself in. She would open it and then roam the house, unlocking other doors and stuff like that. But when she heads downstairs, the toxic fumes that the doll emitted rubbed off on every toy downstairs. Jack-in-the-box's, television sets, other stuff have been corrupted and now are out to kill anything. Your job is to destroy the doll in each level to pass on. Every level the doll got more difficult to kill and stuff. I think it'd be a cool game with all the corrupted toys. Its like an Indie version of Toy Story.
That's my idea. It would probably have a huge background story behind it and some sad and very scary parts. If you could I'd like to do some voice acting.

Also, ending should not be happy.

Well, maybe happy.

But, it should leave the players thinking. And open to theories.

Just like the ending of Portal 2.

OK you need a idea! Lets See! So you nee to surprise people! Find out peoples favorite horror movie and put them in it.That's all I can tell you,sorry Hun

Hey! I, too, am an aspiring game designer. Level design, conceptual art and music are more my thing. If you want to really make a horror game, you should probably look for people to work with who have skills that fill needs on the project! So if you don't think you can write a horror plot and dialogue, you might want to find writers of indie games to work with as well.
I would suggest that a good starting point would be to figure out the mood, setting and gameplay of the game based on what you think you can make. Do you want to make a futuristic, science fiction horror game? A horror game set in a specific time period in the past, like Amnesia? A horror game in the present? Basic questions like this are good to answer first, and you should figure out what kind of gameplay you want to have. Adventure game, like amnesia? Do you want to have combat like Dead Space? In a game, decisions like that affect what kind of story your game is suited to tell, and all these things are questions you should probably answer first and work from there, in my opinion.

What type of story you can tell depends on what kind of game you want to make.

1.A great atmosphere (Various sounds, random events, scripted sequences, good design).
2.No jumpscares is good, but a rare & fitting jumpscare would be nice actually.
3.Fitting ambience.
4.An original concept, or an old concept filled with new ideas.
5.Hidden easter eggs or cool references.
6.A nicely done gameplay with some variety.

The graphics doesn't matter to me, and sometimes, the plot doesn't matter too. The atmosphere is the main thing what makes a great horror game in my opinion.

It takes a lot of time to produce a game like that, but if you do - you'll be most likely the king of the world and get a lot of recognition.