It's quite a smorgasbord, really. A little bit of everything. We have the fantasy aspects - frequently inspired by games, anime, books and other fictional media. Sometimes I will imagine how original content I have thought up would fit into existing games, for example. How the plotlines would change, how the scenes would play out, how the gameplay mechanics would be. Sometimes, I even come up with ideas for my own games or shows, or indulge in something of a derivative of pretend play by 'immersing' myself in alternate worlds in reality. This is the kind of imagination that has been rife in me since childhood. Ofttimes I would entertain myself during boring shopping trips or solitary periods on the playground just by dreaming up all manner of things. The fires aren't as intense, perhaps, but still they burn.
Then there is a more... how would I call it... abstract, almost 'epic' variety. Sometimes I will become inspired by something and think to myself in monologues and prose, often melodramatic and sweeping in nature. I actually find it more helpful to think in languages I can't actually speak properly, such as German or Japanese, because it frees me up from the laborious task of having to find words and I can solely concentrate on the 'feeling' or 'meaning' behind them as it spontaneously unfolds... but if I do think in English, often my best writing is done in these kind of unprompted trance states, which also happen to be the time I am least likely to have access to any writing materials. This came to the fore in my teens, I believe.
Then there are more emotive and realistic aspects. Sometimes I will come up with thought experiments, such as what would happen if I was breaking up with a loved one, or I had just given birth to a child, or I was being threatened by an axe-wielding maniac or something like that, and really put myself in that situation. It's surprising how emotional I can become sometimes in those trains of thought. I have to suppress tears in public, depending! I also contemplate meeting people I would like to meet, write letters in my head, hold conversations. Sometimes I imagine I'm in the real world yet have control over time and space, such that I can essentially fly anywhere, do anything and meet anyone I want, from the past or from the future. That can be fun. Rarely do I seriously contemplate my future, though it is sometimes inspiring to think of all the possibilities - and as I am a hopeless nostalgic sometimes I just enjoy revisiting my favourite memories and reliving them all over again.