I have no idea what Sekai actually wants, but she seems to see the characters as toys to play with. She is even responsible for Futaba's "Toys" event where she forces Sensei, Makoto, and Futaba to have a threesome.
Sekai might have just thought Rin and Otoha having relationship drama, would be entertaining to watch. Overall: Sekai seems to just enjoy being a horrible person.
If she and "The World" are synonymous, then the idea of Makoto finding out her father died, excited Sekai:
She looked forward to a girl finding out she lost her father.