Johnny's mortality

Drac implied in the 1st movie that he'd be willing to change Johnny when saying he's make a good vampire, but Johnny was oblivious and turned it into a joke by imitating him.

BTW, I understand what you're saying about Johnny's character, MiloSuperSplash. They really indicated that Johnny had become what they call (excuse my terminology) p*ssy-whipped. Drac even comments on this during the texting scene. Mavis obviously wears the pants in the relationship. But I think they were trying to imply that Johnny found his balls at the end of it during the family fights towards the end and when he arrived at the battle scene (even if it was already over), or at least knows when to step up if necessary.

Anyway, I do think that either Drac, Mavis, Dennis, or Vlad will turn him at some point. I think they're just waiting until they have to because they love each other exactly as they are, but will eventually do it at some point. I was also thinking with what lampzzt78 said, that turning Johnny could be the plot of the 3rd film. I can see Drac trying to do it secretly because, I don't know, it's against the law or something, or maybe he's afraid Mavis or Johnny's family will oppose because they love him as he is. So here's the plot I have in mind:

Noticing that Johnny is showing signs of aging, Drac starts thinking about the heartbreak he went through when he lost his wife. Not wanting Mavis and Dennis to go through that same pain, he suggests turning him, but Mavis insists that she loves Johnny as he is. Drac decides to take matters into his own hands, maybe even trying to make turning him look like an accident. Enlisting his friends' help, he tries to turn Johnny without being discovered, or maybe even setting up life threatening situations that Johnny gets caught up so the only option left is to turn him or else he'd die. But these plans keep backfiring on him in some way, much like how he tried to bring put Dennis' vampirism. The real problems start when a vampire hunter, perhaps a descendent of Van Helsing, arrives to check out the hotel, so Drac has to also try and make the hunter think it's a human hotel by having everyone pretend their human.
Of course, at some point Mavis finds out that Drac is secretly trying to turn Johnny, causing a big argument. This argument is overheard by the vampire hunter, who realizes that this IS a monster hotel after all, and he tries to kill Drac, Mavis, and Dennis. Just before he shoots Mavis, Johnny steps in to defend her, taking the hit, but they both go down. And as the vampire hunter moves in for the kill on either Drac or Dennis, Johnny comes back, revealing that Mavis bit him to save his life, and defeats the vampire hunter.
After it's all over, Johnny reveals that he's completely cool with being a vampire, and so is his family when they find out.

I don't think he wanted to TURN him into a vampire, he wanted him to BE a vampire. It was more meaningful that way. He loved Dennis whether he was human or vampire, but wanted him to be a vampire. Besides, Mavis would be PISSED if he did turn him. She loved and accepted Johnny for being human, and Dennis (as she believed him to be human), and didn't want to change them into something they weren't. And Dennis wasn't exactly old enough to make that decision for himself. True, he may have wanted to stay at the hotel, but being 5, he wouldn't truly understand what being turned meant until he was older.

Besides, the whole thing about vampires is that they turn humans into vampires. That's the #1 ability all vampires possess. Whether they have all their magic and powers or not, whether they're undead, or a species, no matter what the case, in all media, vampires can turn others into vampires. So to say that vampires can't turn would practically be unethical, random, and not really make sense, especially since there's no implication of it, thus no one would even consider it being a possibility unless otherwise stated.