What often helps me is music.
Soundtracks to be specific. Doesn't matter, if movies, games, TV series. When I listen to it, I usually sooner or later just start imagining my own things over it (if I have a remix or listen to a soundtrack of something I've never seen/played, it also often works very well).
My current game got inspired when I listened to "Hopes and Dreams" from Undertale and "Sanctuary" from Hunchback of Notre Dame, for example.
Writing down ideas (ANY idea) is also a great way, because you may find combinations of ideas that suddenly sound interesting.
For me it sometimes also helps browsing TV Tropes. If I feel like wanting to make a horror game, then I just stroll around the NightmareFuel page until something catches my eye. And when looking at examples (or even just the description text on top), I may just as well get an idea of what to do with the trope (or even go AGAINST that trope).
Of course, consuming media is also great.
Unless you are a caveman from loooooooooong ago, chances are that your idea will not be "build on nothing". But in media sciences, they say that a idea is "original", if it is inspired by other media, BUT those inspirations make up 2%. It can even go as far as that you have 50 inspirational sources where each is 2%. The thing is, with only 2% influence, people will not exactly notice what it is build upon.
If you take Sherlock Holmes, Phantom of the Opera, Girl Genius and Grand Theft Auto 5 and mix that into something, then each makes up about 25%, meaning people will see where you're coming from and probably recognize some elements (just look at movie revues with titles such as "Lord of the Rings meets The Matrix" or "What if Sherlock Holmes would have lived in the 21st century?", those are even blatantly obvious about where they got their ideas from).
It also helps that, if you like a certain piece of media and it is avaible in different medias, that you spread your wings. I loved the "Phantom of the Opera" musical, so I bought the movie from 2004. Then I got the (more brutal and sexual) movie from 1998, I watched the 1915 silent movie and I did read the original novel as well as The Phantom from Susan Kay. Each of these more or less tells the same story, but in VERY different ways.
The musical and 2004 movie are musical adaptions, the 1998 is a horror movie, the 1915 silent movie is a drama, the original novel is something of a horror novel with dramatic elements, The Phantom is even a BIOGRAPHY of the phantom. There is absolutely NOTHING wrong with any of these, and they quote each other obviously very directly.
But if you take one of these and mix them with 10 other ideas, people would lose that connection and your quotes (unless looking for them like people on TV Tropes) and - in the best case - feel like they are watching something new.
Mind you, "new" doesn't equal "good". There is a reason why Hollywood and AAA game titles rehash the same stories again and again and AGAIN. "Don't change a running system", if the only VN with financial success would be Professor Layton, then it would not be shameful to jump on the train and make one like that. If you make something extraordinary and special, you can win big (Amnesia had managed to do that for the horror genre), but you can also fail miserably by alienating potential players (the most famous case may be the E.T. game which took way too complicated gameplay with little developing time and then put it out as a CHILDRENS game).
It is a difficult choice, but chances are, if you REALLY enjoy your idea, if you love talking about it (and - in the best case - if others enjoy LISTENING, then chances are, someone will enjoy playing it. May be a minority, but some fanbases are tiny, yet very passionate. "MUD" was one of the first MMORPGs made in the 80s and even today people love it (it is also what made Richard Bartle research video games, leading to him becoming a VERY famous person regarding online social gaming behaviour, so that game is even from a historical point relevant). And yet chances are nobody here has heard of that game. Not that Trubshaw or the players of MUD care, mind you
Sometimes it already makes a difference to change the genre or gameplay. A Super Mario VN? A Resident Evil Comedy? A Super Meat Boy racing game?
WHY NOT?
So if you have a idea, ANY idea, get it out. Even if you think it is a blatant distasteful copy.
A lot of Skyrim modders got hired to big game companies. Toby Fox (Undertale) had started making Earthbound Fangames and Mods. There is no such thing as a bad idea, there are only ideas that will not result in success (depending on how you define success). I mean, we have nowadays games like "Muddy Heights" where you poop from a roof or "Unicorn Senpai Slap" where you punch a rainbow unicorn that are incredibly popular, so the worst that could happen is that nobody plays it - and the best that can happen is that it makes you lift off ^_^