I'm a Zero fan. though I'm a fan of neither Zeki nor Yumi
err.. back to topic xD
my own guess for the ending is a Yumi ending, be it they live together or die\enter a slumber together.I personally can't see either of Yuki or Kaname be totally happy without the other. especially Kaname, losing Yuki means death to him.
and to be fair,the poor guy lived a long lonely life until he forgot his name. have worked hard and suffered a lot for the sake of humankind since the very beginning.losing beloved one's in the process( HW. along with a wife maybe ? ) thus he deserves at least to have his beloved one in his arms at last be it in life.. or in death.
Zero will be happy for Yuki being totally safe and happy with her Kaname. and will get over his sad\hurt\guilt\pureblood-phobia feelings that he usually get while being around her.
her existence (and blood of course xD) will be enough to soothe his suffering soul. he'll move on with his own life as a hunter and the Head of HA. he'll love a hunter-ess (a girl with a mature body whom will make Yuki slightly jealous.. jk xD ) and live a happy healthy relationship with a couple of cute silver-haired kids and continue the Kiryuu blood-line.
he'll get over his blind hatred toward purebloods and vampires in general just like Cross did. and maybe Cross (along with other characters ) will help him understand that with their death\sacrifice. but still he'll punish without mercy the ones who'll break the law of co-existence.
and I expect Zero's relationship (or at least his opinion) regarding Kaname will progress to the better. though I expect there will be still that familiar funny friction going between them as in Hino's head xD
also I can't see Zero end up with Maria. my guess is that Maria will have something tragic happen to her and ask Zero to eat her so she can join her beloved ones(Shizuka and Ichirou)inside him. that would be a sad but sweet scene between them. her death will affect him more especially if she was somehow defending him with her weak fragile body.
though I'm taking Seiren as an option for Zero xD ( huh? are you nuts ?!) jk xD
whenever I look at this pic.. hmm the one who color it made them a perfect match >> I mean in the looks xD
but seriously, Seiren is still a mystery, no ? I mean her vampire level, her family..etc.