I would rather have a prequel, since I like the ambiguity of the ending, and I'd hate for a sequel to ruin that.  I'd like a series about the rise of the revolution duels.  It wouldn't feature Utena very much, but I don't see a sequel featuring her much either.  Her story is told.  We know who she was and have a pretty good idea of what she may have become.

That said, the series seems pretty complete as it is.  I'd be skeptical of any continuation.

I gotta go with prequel, too. There are questions left unanswered in both time line directions, but you can answer questions about the leadup without destroying the flavor of the series. The ambiguity at the end was so much part of what we loved that to remove it I think would take away a lot of what Ikuhara admits pretty firmly he was going for--the choice of the viewer to see events the way they want to. And after all, that we don't know what happens makes us experience the ending the way Anthy does. She doesn't know what happens either--that's part of growing up. Accepting the unpredictable, the scary, the possible.

A prequel, however, could flesh out various relationships without much changing things as they happen later. Many of the events in the past are pretty much plainly laid out, with some major exceptions that may or may not be done a disservice with detail. Do we really want to know whether Miki and Kozue had sex? Does the extent of their childhood curiosity really matter anyway? On the other hand, Touga's relationship with Nanami and Saionji, or Juri's relationship with Shiori, is pretty well laid out. It would only be expanding on what we already believe to put more screentime into those.

I'm vaguely of the prequel camp myself, as I agree with most of the above points in favor of such.

To play devil's advocate however, while I would appreciate the fleshed out backstory I really I don't think a sequel would ruin the series in my eyes, assuming that it was executed well and at the very least made by people who understand Utena while still knowing what they're doing.

But then, I'm personally against the ambiguous nature of the ending and see the final episode as being more like the close of an arc. The potential to continue the story is too obvious, and my curiosity about what happens to Anthy after leaving Ohtori (and Utena herself, for that matter) can't be sated by mere speculation.

I don't think a sequel or prequel in the conventional sense would work. Improving on the characters as they are would be quite hard, and trying to add more backstory would be more likely to cheapen them than to deepen them. A sequel following the Continued Adventures of Utena and Anthy would ruin the ending of the series for reasons that have already been explained. There are still ways you could do justice to either concept, though. They just have to be farther removed from the plot of the SKU series while retaining its themes. Like so...

A possible prequel: The story of another dueling game held by Anthy and Akio in a different time and place. Maybe France during its Revolution, maybe an American frontier town shortly after the Civil War. Or maybe an American cotton plantation in the South shortly before the Civil War. The race dynamics in this scenario could get very interesting. Akio is the owner of one of the country's biggest plantations, with bold suitors vying for his sister Anthy's hand in marriage. Anyone who looks at him knows he's passing for white, but his money, influence and charisma are such that this fact is politely ignored by everyone - in public. Anthy has a harder time of things, especially when other girls start getting jealous. She dreams of meeting someone who can see what's in her heart rather than her race or her class, but could such a person really exist? And if they did, how would they fare in this world? The shadow girls in this scenario are slave girls making fun of the master and his hangers-on when they're out of earshot.

A possible sequel: Anthy is gone, but Ohtori Academy is still accepting students. Ohtori doesn't admit people in the usual way, though, and as we know, it's not exactly a normal school. You don't have to write essays or get interviewed to go to Ohtori - submitting an application for admittance is more likely to take the form of an act like visiting the grave of your unrequited love, or sneaking into a condemned sanitarium to find the padded room where a once-brilliant artist was held. Ohtori's tuition is always high but never counted in yen, dollars or euros. To make payment for your entry to the school, you'll need to whisper the name you gave your miscarried child, or shed tears over the death of the person you wished you could be. Like a key and a lock the two come together, and a gate is opened to the sunlit garden where no one need ever grow up. Ohtori is not the only place of its kind, either - our world has many points of contact with others, but few people know how to traverse them. These pocket-places have names like the Holiday House and Silent Hill, and most people who enter do so by accident, but there are groups of people who explore them intentionally, seeking knowledge and material leavings of human passion that can be found nowhere else. One day a team of such adventurers happens upon an entrance to Ohtori Academy, but once inside they find it to be far more than they bargained for.

I'd rather have a sequel. It would be pretty tough to pull off but I'd really like to see a show with the same awesome style about the characters' next stage of life. SKU seems to be more about letting go of childhood, I'd like to see a show where the characters (plus maybe some new ones) try to embrace adulthood. You know, in their own messed up ways that may or may not get fixed by the end. I'd like to see Ikuhara sum up adulthood in a giant crazy allegorical university setting or something.

I have to admit that most of the time I hate prequels. They often feel like a cheap way to cash in on an already-written story without having to do the hard work of actually continuing said story in a competent, interesting way. Not that it'd be impossible to make a good SKU prequel but I totally agree with Dani; the irony of SKU delving into its own past is pretty great.

Well, as demonstrated by at least a couple of excellent fanfic writers, it would be possible to make a great sequel to this series, but on the other hand it would kind of destroy the mood set at the end of the series which allows everybody to imagine their own follow-up if an official sequel was actually made happen. Maybe Ikuhara could be persuaded to make a mindfuck sequel which shows a wide variety of alternate events, and then saying that one of them is the true one, but never telling which? I would find that absolutely hilarious.

A prequel would be a lot less risky.  The ending was just superb the way it was and a sequel would have to be just so to not ruin what ended so well.  A prequel could be potentially very interesting, too.  There is so much from the get-go that is worth exploring in terms of the characters, their history, their relationships...I would find the history of Akio and Anthy investigated even more in depth to be highly interesting.  But if a sequel was made that would do the ending justice, I probably would prefer to see that.

When I first saw Utena, I was dying for a sequel. The idea that I'd never find out what happened to Utena or what would happen to Anthy drove me crazy.

But now, many years later, I really don't want either a sequel or a prequel. I think the series is perfected as it is, there is no need to "flesh out" anyone's past more, nor is there any need to explain what would happen after the original series ended.

But if I had to chose on of them, then prequel.