GBJAM's aren't themed so your free to make whatever you would like but we also hope you will respect the jam and it's participants and not start working on the game until October 1st.
Also why is your profile picture of Mo from gamesbymo?
I physically can't start till the 1st so no problem there. I was thinking about porting a game I made for the #SpeccyJam, it would mean rewriting the code + gfx + level design + everything from scratch!