I've always found it odd that they never once made the effort to create a Halloween/Xmas overlay for Minion Mayhem considering they've had the cast reunited for two sequels! They already make short films for the damn Minions (and an unnecessary full length film we shall not speak of). I would've much rather seen two new ride films (hell.... they could pull a Country Bears Vacation Hoedown & have a "summer" overlay as well) for the Halloween/Holiday season. They already decorate the damn area! (wish they'd decorate for Halloween too!) .....meanwhile in Japan..
As for Simpsons, it has been talked that they were once planning on Treehouse during HHN when the original ride film was being made, but there lied to many variables that led it to not happen.
The oddball that should be looked at is Potter, All that I can say is to watch Orlando this year.
^actually the only reason Simpsons never got a Halloween overlay was because Universal said "NO". They wanted to wait until the ride's popularity died

Fox & everyone on that end was all ready to do it. Too bad Universal makes poor decisions. It would've been fun to visit...
In regards to Potter and HHN... yea that is going to be a "NO GO" from the neighbors. It was asked during their MSS panel & WB said they are actively seeing how they can use Potter for their haunt event. So we ain't getting a DeathEater attack show like Japan sadly. (unless maybe it was part of WWOHP's entertainment line-up like Frog Choir/Tri-Wiz)
Isn't it already in place that DWT is going to show holiday-themed shorts during X-mas and Halloween?
I don't believe that's been mentioned, either officially or as a rumor.
During the media preview, it was asked & they did mention its "possible". With that in mind...
Over at HHN Singapore, during the Halloween/HHN season... they change out the SHREK 4D attraction film and show THIS DreamWorks short film.
^The only catch is, they do NOT use the motion seating effects. Despite that, I think this is an awesome way to change up the attraction temporarily & offer a fun non-spooky breather for HHN guests.
Since we have the DreamWorks Animation Theatre, it would allow us to get a fun new pre-show since it is all projection-based anyway (even the display case gimmick). Along with "Scared Shrekless", the OTHER Halloween short film Universal can use is "Monsters vs Aliens: Mutant Pumpkins from Outer Space" (which is amusingly set in California already).
and honestly, this could easily be used in some capacity as well.