I also do not understand this argument. Even in the first game, adding a new hero wouldn’t have changed the fact the the original troup called themselves the U5. No one said that
a) the story is about the U5. The story is about the End times, suggested by the titel of the first game “End Times”. If other characters were to be excluded, what about Lohner? Or Oleysa? Or heck, “CUH-TREEEENA!”
b) Even if it was about the U5, that does not hold any evidence that it had to stay that way. If there was a VT3 (something I highly doubt there will be for a while) and they wanted to have 6 heroes, would they have to come up with 6 completely new ones as to not touch the holy U5?
I can see if you don’t want the vampire girl to be the 6th hero, but what I cannot understand is how, in general, one would be against a 6th hero at all. Getting cool new content is a GOOD thing.
Granted, yes, there’s a possibility a 6th hero might suck. But there was also the possibility the new carreers would suck, and so far, the contrary is true.
My lobbying for Genny-girl is because she would be something different and not just another spellcaster or elf or dwarf or human. We already have two ordinary humans (who have, granted, very distinct and recognizable personalities). I would even agree that getting a 6th hero for the sake of getting a 6th hero wouldn’t be too much of a good idea.
Genny would be a good choice also because it would add a “good” vampire that none of the heroes would have any quarrels with, at least no more than they already have with each other. She has, iirc, saved Karl Franz’s life, so that would put her on good terms with Salty and - to some extent - Kruber. She has settled in Altdorf and thus, would have common ground with Sienna’s Background. She is an essentially immortal woman with a “dark secret” and a “hidden agenda”, which would arouse sympathy with Kerillian. She has been to almost any corner of the Warhammer world, so Bardin might be inclined to probe her for information about “Karak Zorn” (in other words: whatever it is he’s really looking for), and Bardin has been shown to be the most accepting of the bunch (even though his overall interpretation and dialogue makes it seem that this is ALOT more superficial than the first glance might suggest. Bardin is, on the inside, alot less caring and more sinister than his carefree demenour would indicate imho) - and his questionable personal hygiene would allow for some hilarious banter as to why he’d be in no danger of Genny coming over for a “midnight-snack”.
If the plan is truly to support VT2 for 5 to 10 years, a new hero seems almost inevitable, and it should happen within the next year, or else, the player base isn’t big enough for such an investment to be profitable.
Her carreers are, pretty much, set: Lahmian Rogue (Stealth), Blood Dragon Blademaster (Tank / Undead Knights Errant), Von Carstein Blood Knight (Offensive CC akin to Merc), Strigoi Ghoul Queen (melee DPS akin to Slayer or Zealot).
Mark my words: It will happen 
I mean, my other prefered choice would be a high elven spellcaster that had access to a different wind with each of his carreers. That would also be something else, lore friendly, and it would satisfy the crowd who desperately want a second wizard.
