You are not alone on this, this is much more common disease than you think I think there could be several cures, the one that worked out for me was a deadline. When I set up a deadline (very short one) and actually completed a game the first time (exceeding the deadline several times, but that's a minor detail ), I felt so great that I was willing to simply ignore my bloated ego in further games

But I feel like I'm missing out on a whole universe of possibilities.

It's not a feeling, it's a fact. Making games is about sacrifacing 99.99999% of the possibilities. It's unavoidable, do yourself a favour and get used to it.

When we teach at game development workshops for high school classes we always teach them not to make a game. One of our most used (and useful) guidelines is to make a toy not a game. In other words think of a single game mechanic you like, test it, and if it is fun make a game out of it. That way you ensure to always start with something fun, and then you can add story line and other elements afterwards. Preferably as an iterative process.

You are not alone on this, this is much more common disease than you think I think there could be several cures, the one that worked out for me was a deadline. When I set up a deadline (very short one) and actually completed a game the first time

Yes! And the best place to do that imho is at a game jam! Seriously DO IT you don't have to go to a physical one, there are plenty online.

Edit: This technique is probably not well suited for larger game projects, but it is a great training exercise if you want to make simple games.
Edit2: Examples: Gravity gun, Portals, Bullet time, etc.

You can impose limitations. You might limit yourself to a short development time, only use specific tools, not edit code you've already written, or any other limitation that might force you to be more creative. You could also limit the game to exclude certain themes or genres, stick to certain mechanics, only use a limited number of inputs, etc. By imposing limitations you force yourself to be more creative and think of ideas or solutions you might not normally try, and if you're limited to a certain amount of time or certain number of features you will naturally avoid the problem of "feature creep".

A few specific suggestions you could try include:

1. Participate in a game jam or other time-limited competition. You might find these held near your local area, or could participate in them online. A couple of popular examples include The Global Game Jam and Ludum Dare. You can also do this alone rather than waiting for a jam; just set yourself a strict time limit of one weekend, one week, or whatever you feel is suitable.
   
2. Try to create a game using the restrictions of Ernest Adams' Dogma 2001: A Challenge to Game Designers. You'll probably want to ditch the "no credit" requirement, but the technical and thematic limitations (or a similar set of your own selection) will probably force some interesting designs.
   
3. Try Daniel Cook's Game Prototyping Challenges, where a set of graphics and the basics of a design are provided by an experienced designer.
   
4. Try to come up with your own limited ideas, and don't limit yourself to video games -- board games, card games, etc. are great practice too, and come with their own limitations and challenges! Give yourself a couple of tokens and a single dice, and see what sort of game you could create. Try to make a game using only items on your desk. Pick a board game you don't like and try to make it more exciting. Pick a multi-player board game and try to make a single-player version that is still fun and exciting. Make a card game using only one suit from a normal playing deck.
   

Limitations will help to control the scope of your work, and will also force or encourage you to be more creative rather than relying on the same themes or mechanics over and over.



If you feel your current ideas are too derivative you might try expanding the pool from which you draw inspiration, and when you're crafting your own concepts be mindful of the difference between inspiration vs. imitation. It's natural and just about unavoidable that your ideas will be based upon existing material, but you should be careful that you're borrowing, refining and expanding upon one or two key points rather than copying entire collections of game systems. If you only have a limited pool of knowledge to draw from, your ideas will naturally be limited to simple imitations of those existing ideas, so I feel it's important to expose yourself to a wide variety of different concepts, mechanics and ideas, thereby increasing your chance of being able to expand upon some interesting core or base concept in a new and interesting way.

Some specific suggestions for increasing your base and gaining new inspirations include:

1. Research topics you don't know much or anything about. You might investigate a certain period in history, a specific style of architecture, or read the starting text-books from a profession you aren't familiar with. If you don't know where to start, you might use a site such as Damn Interesting (or their book, Alien Hand Syndrome) or try some of the stories from Cracked.com.
   
2. Read books or watch movies from genres you might not normally be interested in. Watch a movie from IMDb's Bottom 100 list, read a piece of classic literature, find a copy of the religious text of a different religion. There are literally millions of books and movies you haven't seen, and some of them might help to spark new ideas for you. If you happen on a movie or book that's just too terrible to actually finish, take the chance to make up your own ending as a creative exercise.
   
3. Try different games. Try horribly ugly flash games on the internet. Try indie games. Try some of the classics. Try to imagine how you might make a modern AAA title work on an older console. Think about how you might update an early 90's game with modern hardware.
   
4. Listen to different types of music, or find covers of popular music done in an unusual style. Try some country music and some death metal. Find out what the lyrics are, and see if you can find out what the original inspirations for the songs were. Make up your own story explaining a fictitious inspiration.
   

You should also read and listen to what other designers have to say, and the techniques they employ. Examine existing games in detail and see how they work. By seeing how existing games were made -- what worked well, and what needs improvement -- and investigating the techniques employed by successful designers, you provide yourself with more tools to approach your own designs. Read The Designer's Notebook, and Lost Garden, and seek out the blogs and published articles of other successful designers. Read about ideas like "skill atoms" and "loops and arcs" that can help you to examine your games in different ways, and check that your designs aren't falling for the common mistakes listed in "The No Twinkie Database -- and then be a bit naughty and see what sort of ideas you can come up with if you commit some of those mistakes on purpose. Read The Pac-Man Dossier and marval at how the simple decisions combine into an interesting game-play experience.


Set limitations. Find new sources of inspiration. Educate yourself on different techniques for design.

Hope that helps!

You might also enjoy Super Mario Bros 3 Level Design Lessons. It's a bit less technical, but takes a look at some of the ways the levels have been designed to encourage or force certain behaviours from the player.