Response to T-RIP
Hey, that's good. Here's what I was thinking: Watts designs a program to destroy Atlas, but the program decides to eradicate all life in Remnant, and manages to stop Salem. A resistance tries to take Atlas back in order to get the Relic of Creation (to attack Skynet). I can't write a story right now, but you definitely should write it if you like the idea.
Response to an unknown guest
I would have to say the Bad Barch would be very creative in their methods of killing Titans. They could definitely help the Survey Corps and fight against Marley (might even see some similarities with the Empire and Marley). Echo relies on technology, but he can still fight. Levi and his squad would definitely watch them all, and I think Eren would want to know about the outside world(s). I could see Erwin agreeing with Hunter, and Hange would definitely love Tech and Echo (she would also annoy the crap out of Crosshair.
Response to another unknown guest
I'd probably have to say reboot. I like how the show has more effects with the TARDIS, Daleks, Cybermen, etc. Also, my favorite version of the Doctor is David Tennant.