Rex Dangervest's comments about having a chiseled jawline hiding under baby fat are a reference to the physical changes voice actor Chris Pratt went through preparing for his role in Guardians of the Galaxy (2014).
During the "Gotham City Guys" song, references are made to every actor who has previously played Batman in the movies: Christian Bale, Michael Keaton, Ben Affleck, George Clooney, Val Kilmer and Adam West.
A comment is made about Marvel "not returning our calls." This likely references the fact that characters from the Marvel Universe are conspicuously absent from the Lego movies, due to rights issues with Disney. Characters from the Star Wars universe (also owned by Disney) appeared in the original Lego movie, but not in this sequel.
Richard Ayoade (Ice Cream Cone) and Noel Fielding (Balthazar) are longtime friends and frequent collaborators having worked together on 'The Mighty Boosh' and 'The IT Crowd' as well as being frequent teammates on 'The Big Fat Quiz of the Year.'
The movie addresses one of the main criticisms of the first movie, i.e. the fact that even though Wyldstyle (AKA Lucy) was the tougher and smarter character and did most of the work, Emmet, the "hapless male" was regarded as the hero.
With the inclusion of his John McClane role in Fox's Die Hard franchise, this is the second that Bruce Willis' role is acted outside the franchise. The first is the mobile game Family Guy: The Quest for Stuff (2014).
Bruce Willis' 4th theatrically released animated film, after Beavis and Butt-Head Do America (1996), Rugrats Go Wild (2003) and Over the Hedge (2006), as well as his 2nd Fully CGI Film with the latter being the first and his first not to be distributed by Paramount Pictures (although the latter film was produced by DreamWorks Animation, it was still distributed by Paramount at the time of release).
The trivia items below may give away important plot points.
Rex Dangervest (Chris Pratt), the main antagonist, says he is a "galaxy defending archaeologist, cowboy, and raptor trainer". To which Emmet (also Chris Pratt) is confused. These are allusions to Chris Pratt's live-action past roles as Peter Quill/Star-Lord in Guardians of the Galaxy (2014), Josh Farady in The Magnificent Seven (2016), and Owen Grady in Jurassic World (2015).
When the Justice League go to fight the alien invaders, someone asks where Batman is, to which Superman replies that he's off on a solo adventure. This, along with comments made by Batman later in the film, seems to indicate that the events of 'The Lego Batman Movie' took place before the events in this movie.
When Maya Rudolph (as "Mom") steps on a brick, she comments that the level of pain is close to that of childbirth. Obviously, she's joking, but studies have shown that the pain of stepping on Lego bricks is actually higher than that of walking across hot coals or broken glass.
In the scene near the wedding, Emmet jumps over the rim of the pyramid shaped planet while Rex stays on it while they're talking. This makes it appear as Emmet is standing at a lake talking to its mirror image, which foreshadows the story, that both are one and the same person.
There is a Unikitty stuffed toy (that can be bought in Real Life) from the Unikitty Spin-Off series from Cartoon Network in the Background of one scene. This seems to explain that Unikitty was a show in this Universe before it was even a show in Real Life. considering the character was original to the Lego Movie brand & how well made the Toy is, it could imply The Boy, Finn, most likely got her from a LEGO Unikitty Box Set instead of making her Himself.
At one point in the movie, Emmet says "let's go save our friends" to Rex Dangervest, but at this point Rex hasn't met any of them yet. Since the audience have yet to learn that Rex is Emmet from the future, meaning this could be a foreshadowing of that reveal.