An Extreme Trial roulette would be completely dead within the first day of its existence. There simply isn't a worthwhile enough reward to encourage people to queue into that content where they fight and people they're with will be entirely random. Think about it. Currently, the highest amount of relevant tome rewards comes from Expert at 90. Even if they matched that. Why would people queue into it when dungeons and hunt trains will be significantly faster with a fraction of the effort?

Roulettes are essentially meant to be a daily chore; something you can easily hop into with zero preparation. Hence why you'll get all types of skill levels. And also why EXs often are a disaster in Mentor Roulette. They're simply not fights you do without a more committed group with a set goal in mind. It won't attract the people who want to try and get their first clear and it absolutely won't attract most who already have it.

Why not make the reward huge then you may ask? Why would I queue in with randoms when I could simply ask friends or my static and we knock it out as part of our weeklies? There's just no way to make it appealing.