Personaly I think it could work and here's some ideas of my own about how it could work.
Well the manga and the ultimate series has great potential. Problem is if it was made into a game it could easily fall into the trap of making the player the all powerful Alucard.
Solution: Make the game centred around the Wild Geese lead by Pip or even Seras as she isn't very powerful until late in the series. There could be sections where you play as Alucard but not all the time and make them a challenge e.g. the fights that could have killed him e.g. Anderson, Walter etc...
There could also be some decent multiplayer. Players play as either members of Hellsing, Iscariot, Millenium etc... Now main characters such as Walter and Alucard could be computer controlled and slogging it out while the multiplayer is taking place. Meaning say your caping the point. Then Alucard gets near you and starts to trash your shit. Now Walter could step in and distract him thus moving the fight away from you. These main characters could be playable like the way it is done in Star Wars Battlefront 2. So I guess the multi would be like Star Wars BattleFront 2 but with buckets of gore.
A cover system could work. There could also be classes I guess. Customisation would be needed e.g. you can make a charater for each faction. Higher level chars get access to say wild geese outifts say. Higer level/top of leaderboard for that game players should get priority over being offered being able to play as main charater for that round.
Maps could have special trigger events e.g. say Seras is impaled agaisnt a wall with bayonets, a player from the Hellsing faction could remove the bayonets thus making her an active part of that round. All these central charaters could be incapped but can become active again after say player interraction (like in example above, after a time limit or even after some enviromental/random event.
Wikipedia page for those of you with no clue as to what Hellsing is: http://en.wikipedia.org/wiki/Hellsing
