The question's come up talking to some writer friends before, which is why this is so detailed; I don't generally run around spinning futile daydreams of potential future success. Also, these assumptions were made before the whole tumblr thing happened, so this whole post operates in a tumblr-fandom-framework.
That said:
DOAMS
Gus would probably be the most popular character, given how he's, well, my most popular character. I could see Elliot as another fandom favorite, mostly because he's so very compatible with fannish interests: Openly gay, fond of casual sex but also deeply values deep emotional connections and takes romance very seriously, a veritable fountain of pop culture references and memes, plenty of opportunity for meta things, looks a bit like he could be a particularly adorable young indie musician but is actually old as balls, which lends itself well to a variety of settings; he's also chipper and upbeat (for the lighthearted romance crowd) but has also seen some bullshit by necessity (for the angst/ and/or hurt/comfort crowd). It'd be interesting to see what ships would become fandom favorites, since the obvious pick of Gus/Jaime suffers from incompatible sexual orientations (but then again, they may just ship it (queer)platonically), as does just about every other ship involving Gus. I could see some variation on Gus/Elliot be popular, though.
There'd probably be a LOT of hurt/comfort stuff, things featuring Gus working through things, case fics, Elliot's exploits in general, and probably ALL the AUs.
Fandom drama would probably arise around a few things:
A) people shipping aroace Gus with likely just about everybody, and endless discussion whether this invalidates his sexuality (A/N: highly depends on how the ships are shipped)
B) Is It Okay For A 4000-ish-Year-Old Vampire To Sleep With Guys In Their 20's If Said Vampire Looks The Same Age: The Never-ending Fandom Wank
C) Is Elliot a sweet cinnamon roll that can do no wrong, a flawed but redeemable person, or just flat-out scum: The Sequel To The Never-Ending Fandom Wank
D) Just... vampires. In general.
E) The inevitable complaints about at least one non-cishet character dying in a story, despite that story not having any cishet characters
F) HOPEFULLY some discussion about Jaime, her unreliable narrator status, and the fact that she's actually kinda terrifying
Equinox
Despite DOAMS generally being my more popular project (because Gus exists), I've gotten some pretty exceptional reactions to Equinox every time I actually got to show off some writing. Which probably means that my character profiles and descriptions for that project suck, but oh well.
I'm actually not sure about the most popular character, because the interplay of two of them is so central to the story - if I'd have to guess (which I do), I'd say Severin, just because he's more upbeat and approachable than Absalom. Then you'd get the Sissy Leonhardt Fanclub (who doesn't even have a profile on here yet, shame on me), some folks rooting for Doc Maur, and some people probably enjoying Valquite a bit, because he's a cute and lovable asshole. The main star of the show would definitely be the budding romance of Absalom and Severin - and therefore also the most common OTP. Then you'd also have Val/Lionelas the second canon ship. I can see people shipping Absalom/Doc Maur for the foe yay, other people shipping Val/Absalom (and instantly getting their asses kicked for it for the age gap and Absalom definitely seeing himself as more of a parental figure in canon - though the ensuing discussions about power imbalances would be fun, because Val's technically speaking the one with a scary amount of power over Absalom), and probably plenty of minor characters getting it on.
Fic-wise, there'd be ALL the AUs, probably, because I can say from experience that Equinox lends itself VERY well to those. Then plenty of hurt/comfort again, plenty of smut, and probably a whole lot of just... assorted romance things.
Fandom Drama:
A) How much of a jackass is Val really, and is it problematic to like him
B) The aforementioned shipping issues
C) Is Absalom actually a kind and sweet cinnamon roll or secretly ~sinister~ after all
D) People shipping the Equinox with Absalom, and somebody getting mad about it because they're not used to good ol' crackships anymore