The most demanded niches according
http://www.penisbot.com/top_listing_secrets.html
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Straight : Celebs
Straight : Close Ups (!)
Straight : Ethnic
Straight : Latin
Straight : Public Nudists
Straight : Softcore (!)
Fetish : Bizarre
Fetish : Blondes
Fetish : Fisting
Fetish : Hairy
Fetish : Pregnant
Fetish : RedHeads (!)
Fetish : Shaved (!)
Fetish : Smothering (!)
Fetish : Spanking (!)
Men : Asian
Men : Bears
Men : Interracial
Men : Latin
Men : Older Men
Men : Uniforms
Men : Video
Other : Bisexuals
Other : For Women
Other : Sex Stories
The niches of the death, we have hundreds submissions there
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Straight : Amateur
Straight : Hardcore
Straight : Lesbians
Straight : Teens
Fetish : Older & Mature
Men : Transsexuals
I don't guarantee the acceptance if you will shift from overflowed to the much rare niches, but I guess the chances will up.