I've just noticed an interesting phenomenom: insert a Western into a non-Western movie series, and the series comes to an end.
Don't believe me?
Back to the Future III: Much of the movie takes place in the Old West. Time finally ran out in this series.
From Dusk Till Dawn 3: The Hangman's Daughter: It bites to say this, but the prequel set in the Old West killed the franchise.
Tremors 4: The Legend Legends: With the Western prequel, the series wormed its way out of existence.
Conclusion: If you want to do a Western, make it self-contained, unless you really hate a movie franchise and want to kill it stone cold dead.