Having already read about a dozen stories in the various "Daria" databases, I've begun thinking about the elements that go into fanfic for this show.
One thing I think is missing from fanfic discussing Daria's romantic relationships (though it was present in the TV show) is an element that must be present in everyone set up as a Daria partner. A long time ago, I knew someone who was as close to being a real-life Daria as you could ever imagine meeting, so the following comments are derived in part from knowing this person.
Not to put too fine a point on it, Daria is a handful. She's brilliant, but she's also extremely intellectualized, and her dating experience is limited. In addition to all the qualities you assume that are necessary to make the relationship work, Daria's partner needs the unlimited ability to put up with her. Reread all the blow-ups that went on between her and Tom in the series, and her spontaneous dumping of him on multiple occasions (particularly her final dump, before high school ended). She thinks through her relationships instead of feeling her way through, so if logic dictates significant trouble ahead, she bails. Her partner needs a massive basic dose of self-confidence and confidence in the relationship, even if Daria dumps him/her/it/them multiple times. Reliability, being there even when there's no apparent reason left to be there, counts for everything in the long run. (Even if she dumps, she might come back later and see if the relationship can be restarted, once she's reconsidered.)
This isn't very romantic, but it's all that will make a long-term relationship with her work. Deeds speak louder to Daria than words.