From time to time, I come up with various game designs. Conqueror: Fields of Victory remains my only published work, but I lots of other projects in various stages of completion.
It seems to me that the ideal game should have some potential for direct conflict between players, use a little bit of resource allocation, including enough randomness to keep things interesting, and be finished in an hour or less.
That last part is key. If a game is interesting but ends quickly, you can also give it another try.
Euchre - the semi-official card game of Michigan - is like that. It's possible to do a couple of hands of Euchre in a few minutes. In high school, people would do a hand or two between classes or a full-on game over lunch.
Perhaps I'm showing my age, but standard kit by my senior year was a Euchre deck in your backpack. Find three other people, and it's game on.
That portability and ease of play was a major factor in the rise of games like Magic: The Gathering and Pokemon. No need to set up a game board, place pieces and so on, just give me some table space and a pack of cards and we can sort things out.
One of George Lucas' many crimes against humanity was his decision to end the licensing agreement for the Star Wars Collectible Card Game. This was a fun and very successful system, but Lucas had by this time bought out Hasbro (which had acquired his original merchandiser, Kenner) and he wanted to consolidate toy and game sales. Hasbro by this point owned Wizards of the Coast, so they were directed at coming up with a suitable replacement that is entirely forgotten today.
At any rate, my mind is turning towards a new game design, and with the holidays coming up, the potential for playtesting is pretty good.