There was an older thread about source material that would never appear in UF, for one reason or another. In this one I wanted to suggest potential crossovers that could appear (even if just a cameo) with minimal tweaking.

Samus Aran from the Metroid series. Given the prevalence of power armor, Samus could show up just about anywhere in UF, but with the Covenant returning as seen in the New Frontier sneak peaks, I think Samus would work particularly well as the first success from a reactivated SPARTAN program. Given the appearance of the Elites in The Revolution Will Be Televised in 2409, the Salusians would have had a few years warning to get ready.

Eddie Riggs from Brutal Legend. Face-melting heavy metal, enough said.

Just have one more at the moment, this one would never show up as more than a cameo. The Crescendolls, the (Gamilon maybe?) band from Daft Punk's Interstellar 5555: The 5tory of the 5ecret 5tar 5ystem (just re-watched it today)