I've seen some nice (but sometimes odd) scents for smoke fluid in hobby shops and at train shows. I really like the Christmas Pine scent - it has that Christmas tree and gingerbread house vibe to it. MTH sells some scents like cinnamon, barbecue, etc. It almost makes me want to eat the stuff (but it's toxic, obviously)!
If I could choose the next three scents to be made for O Scale model railroading, I'd probably go for pizza (everyone likes pizza), soap (you never know; it could actually make your house smell better), and rain (that indistinguishable smell outside after it rains).
What are your thoughts on smoke fluid scents? What would you suggest? ;-)