Thing is, though, that your idea of hard content seems kibda off. I'm not any more experienced in game design, but let's try your way of doing encounters... If I get it right (and I'm truly trying to get it right, correct me if I don't), the main mechanics would be stun and coordination with your group.
Well, the MNK's stun cycle is 6+seconds so, if the mechanics are unpredictable, we need, let's say, 7sec cast. Or people are just doing their stun cycle unsynched with each other... But then, the mnk who has more skill speed will be unsynced as well...
But ok, let's say you have to have a "party optimization cycle" and it works out somehow. That still leaves the "challenge" to be found, really. In a roster where everyone's just listening to the RL, party optimization isn't that hard, but it's a nightmare for pugs... But we still want hard content for both. Still, how is it hard when healers just spam heal, and tanks just ensure they don't die ?
Maybe you don't like telegraphed attacks and dps checks, but those are two mechanics that combine really well, because, when you have to run around and coordinate with your party to avoid things without killing each other, you can't deal damage... So you have to either be even more coordinated and make minimal movement, or have a perfect rotation to optimize your dps. That's not hard in a "bullet hell" way, not in a "monster hunter" way or whatever, but that's hard in a "mmo" way, I guess...
Plus, you need to consider that although the "optimize dps and avoid tellegraphed attacks" is roughly the same every time, you can't compare the mechanics of O3S and O4S, and so on. They're actually doing a pretty good job reiventing themselves...