With Doom, of all things. The most likely explanation is that both involving dealing with a glut of corrupt demons, albeit through comicallyopposite methods. Either way, there's a lot of fanart that shows Doomguy and Charlie getting along.
Many fans of Satellite City are also fans of this show, and vice versa, thanks to both being web series' about a dysfunctional group of Creepy Cute monsters and featuring tons of dark humor. The fact that, by sheer coincidence, Satellite City's Halloween video for 2019 was uploaded on the same day as the HH pilot just added to the crossover appeal. The fact the Hazbin Hotel Amino account has expressed they enjoy the thought of a crossover also helped a lot.
There is also a minor overlap with the Welcome to Hell fandom, largely due to the similar themes but polar-opposite execution. Some people are already trying to wrap their head around what Sock and Mephistopheles might look like in this show's universe.
There's also overlap with the JoJo's Bizarre Adventure fandom, partially because of the strong LGBT elements in both fandoms. Mick Lauer has also been involved with both shows, voicing Leone Abbacchio in Part 5/Golden Wind and Husk in Hazbin Hotel's pilot.
Many fans of the show also like Villainous, due to the similarities in the art style and the similarities between Black Hat, Lucifer and Alastor. It helps that some of the staff of Hazbin also worked on Villainous.
Fans of Gravity Falls also quickly took to this show, mostly due to the similarities between Bill and Alastor.
The fandom has managed to get along well with the fandom of She-Ra and the Princesses of Power due to similar themes of redemption, gender identity, abuse, among some. Not to mention that some find Double Trouble to be similar to Angel Dust.
While not as vocal, some Duck Tales fans have recently got into the series as well due to some Hazbin fans finding Louie a bit similar to Angel Dust (personality-wise).
There's also some with The Owl House. Both feature a quirky bi girl as the protagonist whose love interest is a moody lesbian who would do anything for their partner, and who have a friendship with a tall, insanely powerful being with an affinity for the color red, a sentient staff/cane, a Nightmare Fuel inducing demon form, and a distinct Animal Motif. At least one DeviantArt artist even made a headcanon of Eda and Alastor secretly being siblings.
Weirdly enough, there is a lot of overlap between Hazbin fans and ULTRAKILL fans, though this can maybe be chalked up to both having large LGBT Fanbases and prominently featuring non-standard depictions of both Heaven and Hell, though ULTRAKILL is obviously much more cynical and nihilistic with its version. ULTRAKILL fans also point out the hilarious coincidence that the first episode of Hazbin Hotel and ULTRAKILL's prologue layer are both called "Overture".
With Star vs. the Forces of Evil due to Charlie and Star being extremely similar in design and personality, as well as both being princesses that aim to end a longstanding status quo among their people. There's also some overlap with Tom, who's a demon that dated Star, while Charlie is a literal demon that lives in Hell.
With Helltaker for a series of lovable, attractive demons and some of them bear similarities.
