my own experience is not necessary about what game idea, its more about what functions id like to learn, or genre to have a go at, or just anything that i would like to experiment with. The game idea usually depends on that and i would generally just go with whatever flows out at the time, it kinda comes together somehow, not always successfully. but what's important to me is what i learned during the process, not about producing a great game demo. Sometimes breaking old habits and trying something new can trigger lots of different ideas, that you can either go with and start something new, or apply it to something you have always done or more comfortable with. Try looking at other old game jam themes, they could inspire, magazines, why not have a look at the OGA Game ideas thread? How about makiing a tool instead of a game?, visual novel? Trading card game, Flofflewoffle top trumps using all your furry characters. :)