Curate, connect, and discover
I won't call this an unpopular headcanon, just one that is slept on.
So, everyone knows about Alastor making deer noises, both in canon and fanon. In the show he canonically makes little fawn squeaks when Rosie hugs him (which lives in my head rent free at all times) and does the elk scream a couple times throughout the show. Fanon is just as up in arms about this trait of his, and I think it's great.
But.
What if he ALSO made alligator noises? HEAR ME OUT - trust me, you won't regret it.
So.
When you really think about it, there are a large amount of traits that Alastor has that would make it logical for him to share traits with an alligator as well as a deer. These include: teeth/habit of eating sinners(cannibalism) (alligators are known cannibals), place of origin being Louisiana, persistent presence of bayous in his life (near his home in the living world, supposedly where he hid his victims, where he died, and the bayou in his room), ambush predator (makes you feel at ease by being unassuming then strikes before you even realize the danger), and his temper (appears calm until he's NOT) just to name a few.
Despite what people tend to think, alligators actually make a TON of different sounds, though there's a few main ones I'll talk about.
First one is the territorial growl.
I feel like Alastor could make this sound when he's being territorial (obviously), threatening, or protective (as rare as that is). He begins making this sound a lot more often the longer he stays at the hotel because he begins to see it as his territory, and the residents as his people (not that he'd ever admit that lol). Also, I feel like hearing this loud, bass-feeling noise from this rail-thin deer man would be a bit jarring, especially if you'd never heard it before
Second sound is the defensive hiss.
This is a sound Alastor would make if he's feeling threatened. I'm thinking scenarios where he's annoyed/mad/anxious are when he'd hiss, because they're all scenarios where he'd be trying to get something or someone to leave him alone. It's a very different yet very similar sound to a cat's hiss. They can mean similar things, but hearing an alligator hiss is significantly more threatening, which fits Alastor's character. Also, it means he can still sound terrifying when he's the one who's scared (if he manages to make a distressed gator sound rather than a distressed deer sound that is lol)
And of course, we can't forget the mandatory cute sound, the baby swamp puppy chirps.
If he gets to make fawn noises, you bet your ass he's gonna make swamp puppy laser sounds. The scenarios where he'd make these noises are a lot more rare, but they definitely exist. He wouldn't ever admit it, but he sees Rosie as a mother figure, and just as he tends to greet her with fawn squeaks, Alastor will subconsciously start doing little baby gator chirps when he's looking for her and can't find her. His instincts are to call out to her so she can find him - the same way baby alligators do with their mothers - and the side effect is that he makes cute little laser chirping noises without even realizing it. He has a tendency to do it when he's looking for anyone he likes who is older than him. Zestial finds it charming and the few times it happened to him, Husk refuses to talk about, a lot of mixed feelings brought up by the fact that the man who owns his soul has given him at least somewhat of a parental role subconsciously.
anyways, these were just my thoughts on it, feel free to add on your own or use it as inspiration for your own work (let me know though, I want to see it!!!!) (putting this in the lee!alastor tags to feed the imaginations of my friends there who are always starving for more things)