Talking of keeping DPS and healing engaging. I've decided to monitor what I'm doing the last couple of days - just doing roulettes, because they're a daily thing. I worked out that on my SCH, which isn't that well geared my downtime looked like this:
Expert Roulette (The Twinning): ~45% downtime (average pulls)
Expert Roulette (Amaurot): ~35% downtime (high to full pulls)
Levelling Roulette (Sirensong Sea): ~55% downtime (average pulls)
Trials Roulette (Susano): ~30% downtime
Susano is a trial that has more raid wide AoEs and also hug mechanics and mechanics people easily get clipped on. So this meant I healed more. Plus IMO SB trial encounters were well designed. But the healing aspect was still very straight forward.
Amaurot: The tank did good pulls, not quite wall-to-wall, this was little more engaging than those who don't, especially towards the end of the dungeon where the pulls are bigger and deadlier. But on the second boss I was doing like 65% Art of War, that encounter wasn't engaging as a SCH. Healing again, was super easy, less so at the end section with the wall-to-wall pull - that's the only point I felt I was doing something as a healer and this was brought on by having to move around a lot (lots of AoE's), people getting hit by AoE's and the tank taking a good amount of damage because he's got everything he could pull.
But I feel given the lack of variety in our DPS rotations, 30+% is a high margin of downtime. Plus I only consider myself or medium to high skill, I don't even do savage and I wouldn't rate myself highly. But 30-35% still can be kinda engaging depending on how it's spaced out, but when it gets past that, then I find it's not.
See this is the kind of thing I think something like GW2 does right. Early on you get the core and defining skills of your class, then you learn more as you progress. It meant I could get stuck in pretty early and get an idea of how something plays without investing too many levels into it. And at the same time, you have abilities on top you can invest in picking up. Though their system of course is very different to FFXIV, but I think at least the principle here is compatible. Low level stuff may not necessarily have to push you to use your full kit because you might still be learning at a lower level, but then in MMO's in general, lower level stuff is forgiving anyway to accommodate this.