Good question Temptress but in my opinion, once you get into those unique fetish niches like feet and stockings and smoking... I think there is no single one that stands way above the rest. I personally try and push every niche I can find content for on my sites and there is no one winner. I know this really doesn't answer your question but maybe someone else will have some better insight on the subject than me..... but my advice is not to limit yourself to one or two other niches besides your prego site... I say go for them all
P.S. I see upstate N.Y. in your profile... where abouts?? I was born in Rochester many moons ago. I loved it up there but couldn't wait to get out... if that makes any sense