Two insanely powerful warriors who've killed hundred, if not thousands of the enemies, and are the best their organizations have to offer and usually tasked with eliminating their own kind (Cyborgs and Vampires) for the safety of humanity.
They would both serve some sort of company (Hellsing Organization and various PMC's)
Both would also go by a different name based on a old European legend (Dracula/Vlad The Impaler and Jack The Ripper)
Both were also regular humans before turning into what they are today (Vampires and Cyborgs)
They both also have a restriction within them that when released, reveal their true power at the cost of making them slowly weaker (Alucard's Level 0 removing every soul and weakening some of his abilities while Jack's ripper mode slowly drains his power cells)
They're moral ambiguity is shown on full display at times seeing as they're both condemned to a life of violence and death, hiding much more blood thirsty and slightly sadistic tendencies yet still using these morals to protect mankind.
Both would gain many, many titles from their deeds and regarded as heroes or monsters by many.
Both also use their previously defeated and dead enemies to help them fight and gain more strength (Raiden using various weapons of his enemies and Alucard's familiars).
Science vs Magic
Holy fuck, there's so much that can be done
There can be a swordfight between Alucard and Raiden.
A gun fight with Raiden putting his old MGS weapons to the test against Alucard.
Raiden using one Monsoon's Sai's only for Alucard to use his telekinesis in response.
Alucard turning into multiple bat's and Raiden activates Blade Mode to accurately cut each of them
Have Alucard send out his undead army as Level 0 as Raiden feels despair... Only for him to start laughing as he turns into Jack The Ripper and charges straight into battle and start hacking and slashing each and every one as a remix of Stains of Time, Red Sun or A Soul Can't Be Cut plays.
Alucard summoning Baskerville as Raiden hides and uses stealth to get a sneak attack and cuts it up or blows it up, that or he starts fighting in from a distance by using his polearm
Having the fight end in a blood filled and thunderstorm night would be a fantastic thing to look at, with the sounds of one them overpowering the other depending on who's winning
Bruh just imagine hearing Atleast one of these while Raiden goes on a one man army killing spree against Alucard's undead army
Hey, remember that creepy huge arm thing that Alucard used in Alucard vs DIO? Imagine bringing that back and using that again for a Rules of Nature moment
Alucard starts firing off his guns at a distance, only for Raiden to rapidly approach with his Ninja Run as he runs and deflects the bullets away
A Moment of Raiden actually struggling against the sea of blood as a QTE pops up on screen for him to keep pushing through