That's illegal as hell in Nevada, and I would assume in most jurisdictions in the US that permit casino gambling. The machines are very, very carefully engineered to give exactly the odds reported to the regulator. If you don't, that's a corporate death sentence; if Nevada refuses to certify your machines you can't sell them anywhere.
The difficulty of certifying a machine / game / etc is so high that the industry mostly uses a few basic games which are proven (both from a legal and operational perspective) and then reskins them endlessly to keep players coming back. Vampires were in for the last few years, apparently, thanks to Twilight. (Anecdata based on walking through a few casinos while in town for Microconf. It's vampires, TV shows appealing to older Americans, and a lot of very very obviously US-company-attempts-to-do-China games.)
The difficulty of certifying a machine / game / etc is so high that the industry mostly uses a few basic games which are proven (both from a legal and operational perspective) and then reskins them endlessly to keep players coming back. Vampires were in for the last few years, apparently, thanks to Twilight. (Anecdata based on walking through a few casinos while in town for Microconf. It's vampires, TV shows appealing to older Americans, and a lot of very very obviously US-company-attempts-to-do-China games.)