I have been binging both franchises for a long while now, and seeing that I like crossovers, a thought occurred to me: why hasn't there been a crossover between these two series? They seem perfect for each other. They both revolve around humans battling monsters in the night after all. This has more than likely been thought of before, but I wanted to try and give my 2 cents on this idea.
Idea 1:
Yoriichi Tsugikuni teams up with Trevor, Sypha, and Alucard. They all lived in the same time period, so it wouldn't be too farfetched for them to meet naturally. Yoriichi lived during the Sengoku Era (1467 to 1615) while Trevor's story was from 1456 to 1479.
Idea 2:
Alucard lives all the way to the events of Demon Slayer. He joins the Demon Slayers as an ally against Muzan while also being vigilant for Dracula's return. However, I can see him being in rough terms with the Pillars like how it was for Nezuko. In terms of romance/pairings, I would ship Alucard with either Shinobu, Kanae, or Tamayo. My reason for Shinobu is because like Alucard, she is someone of science and knowledge when it comes to monsters. Seeing that Alucard holds much of his parents' research within the castle, he could help Shinobu expand her horizons like Dracula did for Lisa when they first met. Also, adopted from her sister's dream, she believes that humans can live alongside Demons. Alucard, being part human and part vampire would be someone of great interest to her. It could be living proof of her sister's dream (I know that Alucard is a vampire and not a demon but still...the concept can still work). My reason for Kanae is becauase she believes that humans can coexist with demons. Alucard is hybrid of humans and monsters, so that would be something that I know she pay much attention to. Also, her kind personality. My reason for Tamayo is because for one thing, she is someone who does research on demons/monsters all the time, so Alucard's vast knowledge could help expand her horizons like he could for Shinobu. Another thing is that they're both monsters that aid humanity against the darkness.
I can also see Muzan being interested in Alucard. He is vampire that can walk in the daylight. Neither demons and vampires can handle the sunlight, so I know for a fact that this would be something for him to keep his eye on. He might even try absorb Alucard like he wanted with Nezuko.
Another idea that comes to mind is that Alucard begins to develop his alter ego, Genya Arikado, to blend in with the populace.
Idea 3:
I think some Castlevania characters should be a combination from both the animated series and the games to give them more of a chance. I don't deny that they are skilled in the Netflix series, but their game counterparts have the most variety of abilities (especially Hector and Issac). Heck, Hector never even fought in the Netflix series. They have the gadgets and abilities to kill demons but demons from Demon Slayer (particularly Muzan and the Upper Moons) are far more skilled in combat; comaparable to the strongest vampires if not more so. I believe that they would need to have more physical attributes to keep up with them.
The strongest Belmonts known in the Castlevania franchise are Simon, Richter, and Julius, so I think Trevor could work his way up to reach their level.
Idea 4:
Trevor, Sypha, and Alucard teleport to the world of Kimetsu no Yaiba.
*In addition to his family being a clan of monster hunters, Trevor has the Vampire Killer: a weapon that was made to kill monsters of the night. That's pretty self explanatory. I can see him getting along with Gyomei, Rengoku, and Tengen.
*Sypha's fire magic would be a great way to amplify Nichirin Blades (turn them red so to speak). Additionally, her ice magic would be a good counter against Doma's Blood Demon Art. I can see her getting along with Shinobu, Mitsuri, and Tamayo (Shinobu and Tamayo particularly since they're both knowledgeable)
*Alucard is pretty self explanatory as well. I can also see him getting along with Shinobu and Tamayo when it comes to science and understanding demons/monsters better. Would probably remind him of his parents. Like Idea 2, he could help expand their knowledge with his own family's knowledge. I can also see him being interested in Nezuko. Like him, she is a monster that helps kill other monsters.