I'd recommend finding some movies or tv shows that you like. That can be anything from design wise to story line wise. Figure out the basis for what type of slot design would work for you (something like Gates (4 scatter) or Big Bass (3 Scatter). Write down EVERY idea you have and put them into a list. Also, write down the base functions for the game such as how many symbols are in the base game, how many are needed to connect, the pay scale, etc. After once you have a lot of that set in stone for a rough final draft, focus on the art factor behind it all. Beyond that, I'll be honest- I'm not too sure lol.