Three Challenges
Challenge #1: Tarzan / Planet of the Apes: Tarzan and Jane (you pick the version: the book, Disney, live action movie, etc) somehow find themselves on the Planet of the Apes (again, you pick the version). Maybe an enemy used a ritual or spell to warp them away or something. How will Tarzan, who was raised by and loves apes, deal with apes that use guns and enslave humans? Especially if they separate him from Jane?
Challenge #2: Nasuverse / Perfect Half: In case you don't know, Perfect Half is a manhwa set in a world where the genders are divided and go to war for some reason. The men and women usually never see people of the opposite gender except on the battlefield, when they are captured for reproduction purposes, or when they call truces for reproduction purposes. The main character Haerang is a rookie male private who gets captured. He impresses all his captors with his sexual stamina, including Ahrah, the strongest woman in the world, much to the jealousy of Karim, the strongest man in the world who desires Ahrah. After getting rescued, Haerang proves himself a formidable warrior in his own right and slowly builds up a harem while his comrades also build up romances with their opposite numbers, all while conspiracies involving the royalty of the male and female kingdoms lurk.
Anyway, what if the Nasuverse was divided by gender and at war? There could be a squad consisting of people like Shirou Emiya, Shiki Tohno, Mikiya Kokuko, Hakuno Kishinami, Sieg, Ritsuka Fujimaru, and/or Miyamoto Iori who then fight and eventually romance their respective girls.
Challenge #3: The Magic School Bus / Wayside School: Either Ms. Frizzle and her class move to Wayside School, or Ms. Frizzle is a substitute teacher for Mrs. Jewls' class. Either way, things are going to get weird.