One of the other holiday exchanges I take part in, xmas_rocks, allows participants to choose a pairing and then nominate additional peopleso if the author is comfortable writing a threesome or moresome and they're familiar with the characters, they can, but they're only required to write the pairing requested. Reading your first suggestion, I wondered if something along those lines could possibly work? So the author volunteers to write any threesome in, say, Doctor Who, and the recipent requests Amy/Rory/Eleventh Doctor, and then says that if the author is comfortable including River Song, that would be great. That way there is a chance for the OTplus, but no one is obligated to write something they can't do. Just an idea for a more flexible way to handle that.