Serena is certainly by far the most well developed character in skyrim and is the best candidate for a wife in skyrim. she has an amazing personality, great background story and when you play through dawnguard with her you actually feel an emotional bond growing between you and her and a sense of trust. she is not mariable however Bethesda are by far not stupid. for them to not let you marry her is a sign of stupidity considering that thousands of fans have been disappointed by the fact that you cant marry her and ignoring this would certainly be stupid. therefore in my personal opinion this suggests that Bethesda have got something planned. maybe a new quest line involving Serena? after all she as a character has been such a great success in dawnguard that Bethesda feel they should centre an entire patch or possibly DLC around her? time will tell however its such an obvious thing that Serena should be mariable therefore I feel that an entire company couldn't have possibly overlooked that! that leads to only two possible conclusions. 1. Bethesda is trolling us however its a mistake as it upsets a lot of fans. 2. which is likely, they have something in store for us to do with Serena. maybe a new DLC? quest line? maybe at the end you can marry her? maybe she dies dramatically? maybe she decides she doesn't want to be a vampire any more? maybe the quest line mgiht involve her trying to escape molag bal's grasp and curing herself so you might have to defy a deadra lord which could potentially have a juicy storline Bethesda can take advantage of. i don't know, these are just my opinions and speculations. its a matter down and open to debate however I just wanted to share the idea to the community that not being able to marry Serena is deliberate and maybe Bethesda are just holding back on us for now.