One could argue that Spock's World brings in the entirety of Doctor Who as an in-universe work of fiction.
Except that My Enemy, My Ally
(or was it The Wounded Sky
?) had previously depicted it as a work of fiction. One of the old 20th-century videos that Ensign Freeman and Uhura were converting to 3-D was shown playing, with a blue box materializing and a curly-haired fellow in a long, colorful scarf emerging (and saying a line Tom Baker never said in any actual episode).
I don't recall how Spock's World
references Doctor Who
, but Ishmael
certainly alludes to it with references to Metebelis crystals and a time-travelling civilization in Kasterborous (Gallifrey's home galaxy).
I read somewhere that at some point in the DS9-R Taran'atar picks a fight with an Aliens Alien in the holosuite, so the same applies.
All that proves is that the Alien
franchise exists as fiction in the Trek universe. It's no different from Data playing Sherlock Holmes or Henry V.
And isn't Predator a part of the same universe? Considering the Jem'Hadar are fairly blatant Predator rip-offs this is quite interesting.
Actually the Nausicaans are the ones who resemble Predators.
I've read that Ishmael had one or two big huge unauthorized crossovers (to stuff I didn't watch so have no knowledge of), too.
The whole book is a crossover with the Western Here Come the Brides