X-Men/Lord of the Rings (Just to see Kitty Pryde and Lockheed in a fantasy setting).
Robocop/Detroit: Become Human (Murphy and Connor Cyborg-Android team up for the win!) In fact let's throw in I-Robot, Westworld, Blade Runner and A.I.D.A from Agents of SHIELD whilst we're here. The Androids have spoken.
Doctor Who with just about anything (There's a reason it tops fanfic crossovers).