The following contains minor spoilers for The LEGO Batman Movie. Stop here if you wish to avoid them.
The finale of The LEGO Batman Movie has a whole lot going on in it. The movie brings in pretty much all of Batman's Rogue's Gallery. However, there were a few characters that the director wanted to add who he wasn't able to bring in. Since the list of iconic villains in the film extends far beyond those associated with Batman, it seems pretty much anybody could have shown up, including villains from such films as Misery and Kill Bill.
I would have had Kathy Bates from Misery, and [Sherlock Holmes nemesis] Moriarty, and at one point I pitched Daniel Day-Lewis's character from Gangs of New York and David Carradine from Kill Bill.
While Director Chris McKay admits that getting the rights to use so many villains was a chore for the movie, the reason that characters like Moriarty and Bill "The Butcher" weren't used had less to do with being able to get the ok, and much more to do with the fact that a large segment of the audience, largely the kids, wouldn't have gotten the joke. In addition, he tells Entertainment Weekly that when you're transforming all of these characters into LEGO minifigs, it can be difficult to create a look that's able to express everything you need to express in just a look. With the sheer number of villains seen at the end of The LEGO Batman Movie, the vast majority don't get a lot of screen time, as such, how do you create a LEGO version of Kathy Bates from Misery that makes it clear to everybody that that's the character you're looking at? As cool as it would have been to see Moriarty make an appearance, that character, being born of literature, doesn't really a visual representation who would be universally recognized. To be fair, though, a Bill "The Butcher" cameo probably would have worked out, that's a pretty iconic look. As long as the mustache makes the transition to LEGO, you're fine.
The other reason that some characters who were originally considered were left out, according to Chris McKay, is that he was worried they weren't doing enough with the characters they had. This was The LEGO Batman Movie after all, as such, you don't want to overshadow the Batman world with too many unrelated characters.
We're guessing there will be plenty of additional cameo opportunities for more villains in the future. The LEGO franchise appears to be going strong so there's a good chance that Batman will be getting more screen time in the future and thus he'll need more villains to fight.