How about Passover? Nobody ever suggests Jewish Holidays ;) You could have a Die Die Anu (phonetically spelled (not sure how to spell phonetically though)) as a dark ride like Small World in WDW. And Mozeltov could be a Zamperla Rotoshake. You could really get into that, even include the different plagues, it could really be fun!
Easter would be a good family addition, but hard to theme additional rides to IMO. Valentine's day would be interesting, but possibly also hard to theme. Plus a whole landscape of red and pink could possibly drive a man insane. ;) I can't think of many things to associate thanksgiving with besides turkey and the first thanksgiving (indians, settlers, etc.). Maybe I'm just not original enough to think of much more! Personally, of all the holidays mentioned, I think St. Patricks Day would be the easiest to add, easiest to theme rides to, and fit well with the existing Halloween, Christmas and 4th of July.
To throw a whole new twist in the idea, where would a new area be added? Without taking land from existing areas, and assuming that the land the park is currently using is about all they land they have for expansion, the only space I see left is the grassy area between Raven and Legend. Ideas?
A Jewish holiday section in Southern Indiana? Free yarmulkes? ;)
St Swivven's Day...weird, there's a word (name) I know for a fact I cannot spell....never seen it in print.
I like Festivus, but again, prob. not "appropriate" for the park, likewise for Mardi Gras...Memorial Day and Labor Day seem more likely, or Thanksgiving (a turkey-shoot themed dark ride mayhaps?)...of course Valentines Day opens up the possibility of a Sally-brand interactive with a bow-and-arrow, LOL....:)