I do see the capacity for at least two more expansions and a lot more filler content.
SPOILERS UP THE FUCK AHEAD. STFU AND GO FINISH THE STORY BEFORE READING ON.
The first thing is to actually complete the Zealot path and remove some of the deaths when you go down the rabbit hole with your Zealot friends.
The second I could see right after the Professor leaves you. Instead of doing anything he wishes, you live among the humans. You don't care for the Church, the Wolves, nobody, except for the actual laymen. That, or some other interaction with humans (perhaps you see the caravan earlier on and live with the nomads?)
Filler paths with Darla and Cecilia. Cecilia, to me, appears just like a thirsty bitch (literally) fascinated with your being a vamp. Perhaps you could play on a bit more with her obsession and make her more insane if you didn't turn her into a ghoul earlier, or perhaps you could up your relationship with Darla and piss Cecilia, causing her to go against you at some point in time.
You could make optional paths actually influence how characters work. Gideon could avoid trying to fight you, or you not talking with Ragnor could possibly piss him off when he finds out you're ghouling people in the city. Or was it Gideon that liked people?
Filler plots with Allen and other young-bloods. I didn't really care when they died, tbh. Just a little "oh, look this n00b got kickscopp'd, k den" and then you're back on the path of revenge.
I could also suppose you could follow the Professor's word much earlier on and off him even before you part from him and put on a totalitarian regime style of rule where ghouling is a privilege and requirement for joining your nihilist party in your epic quest for making another perfect vamp to succeed you.
Alternatively, you could try and fail to preserve everyone, bonding and losing everyone you know and love gradually to their inherently unsociable and extremist ideals.
There's a lot left in this world to explore, really.