The Premise here is simple: One day in Nerima a strange artifact is unearthed that has strange, unearthly powers.
Thieves attempt to steal it but are thwarted almost as an afterthought by Team Ranma, who just happened to be in the area for no reason in particular. The artifact is dropped by one of the bandits and is picked by Ranma. Cdologne recognizes it and warns him not to tamper with things he does not understan, but naturally enough Ranma accidentally breaks the seal on the device and is sucked into a void that transports him back to another time, place and era... Ranma trapped as a girl finds herself in a very strange land (which he quickly deduces is no where near Japan, let alone Tokyo for that matter) where he meets none other than Count Dracula.
Hellsing is named after and centered around the Holy Order of Protestant Knights, originally led by Abraham Van Helsing. The mission of Hellsing is to search and destroy those that threaten the safety of the Queen and Country from the undead and other supernatural forces of evil. This organization is currently led by Sir Integra Fairbrook Wingates Hellsing, who inherited the leadership of Hellsing as a child after the death of her father. She witnessed his death which turned her from a once innocent and shy little girl to a tough and deadly force. She is protected by the faithful Hellsing family butler and former Hellsing "trashman" Walter C. Dornez, a deadly foe in his own right, and Ranma Saotome, a most powerful vampire that swore loyalty to the Hellsing family after being defeated by Van Helsing one hundred years before the story takes place. These formidable guardians are joined early on in the storyline by former police officer Seras Victoria, whom Ranma turned into a vampire.
What if in "Ranma1/2" universe Ranma become locked as a girl and somehow send back few hundreds years to the past, where she become a vampire ( or curse is locked when Ranma is bitten by vampire while he is a she) and later is "tamed" by Hellsing Organization. Then we can think up few stories (from Hellsing involment during world war two for example) as a filler/extended prologue. Then(In our times. take 1 year from Ranma's disappearance) we can go for main plot where Ranma gets a mission where she must travel to Japan(Nermia). There she must not only fight enemy but also deal with her past, family and friends. Is she still human enough to do this, what they think of this "new" Ranma.
It starts in "Ranma1/2" timeline, maybe even some before Herb( or after, what would be better?). Then bring Ranma to the past, write one or two (or more) episodes in there, after that speed time to the beginning of "Hellsing". There will by changes. After all Ranma, no matter how changed, is still Ranma and she (Yes I intend to left her female, it would provide few possible situations with Nodoka later on) would do much more to try save Seras then Alucard. After all that I intend to set my own original plot with Ranmas arrival in Nermia a year after HE disappeared.