The main evil character opens a shop and sells "Needful Things" which people covet but can't afford. So he makes deals with them to pay for the "things" and gets all kinds of mayhem going. Not sure how you could work that, as it is more of an interplay than I realized when I posted before.
But I think you could combine sci-fi and horror into the mad scientist room via dim green lighting, lots of "lab" tables with fake stuff on them (tubes, bottles, you know), sound effects on a tape player (burbling, slurpy kind of sounds) and maybe some strategically placed light flashes. A lab skeleton with intermittent flashes of backlight, and there are lots of crafty ways you could make the "experiments". You could use those tubed lights they sell in holiday colors (I'm sure they'll have Halloween colors available soon, if not now) to run from a few "Bottles" to represent some yucky liquid flow!
A lot of this you could make yourself, including taping the sound effects! And keep it relatively dark.
Designing sets and props was my favorite part of my years studying theatre. Much more fun than acting, for me. In fact, if you have enough people, you could stage a ghoulish wildhaired scientist, too!
BOOOWHAAHAHAHAHA!