The issue with the "require more healing" route is that we already do have content that requires more healing. It's ultimate, and even that is a meager 20-30% healing. But it's already way beyond the reach of many.
In reality it's not the amount of healing required that needs to change but the damage distribution. You would need to spread damage in more random damage spikes as opposed to the choreographed encounters we currently have. The problem? SE are going the exact opposite with their design. They've removed critical hits from bosses, etc. etc.
This, like you've said, breads "lean" healing strategies where a good healer is measured by how little he can overheal and how much he can dps as a result.
You mentioned Sunwell earlier in the thread and that was kind of the opposite end of the spectrum. Promoted heavy healing where a good healer was measured by their ability to consistently overheal and not go OOM. Effective healing although important was relegated to the backseat since most healers were usually segregated to specific targets/groups. For those wondering, damage output was random, constant, and heavy. A lot could happen between the beginning of your cast and the end so keeping your heals rolling was a must, even on full health targets.
All this to say that there's got to be some middle ground but SE would need to change the way damage is dealt and alter mana management to go along with this. The main issue is that like other have mentioned. This means that the mana reworks for future (diff damage pattern) content would break past (current damage pattern) content and that will not slide so is unlikely to happen at all.
At this point everyone should realize that the dps meta we currently have is here to stay and healers should just get more diverse dps tools. That's all there is to it. SE can make a new MMO down the road with the lessons learned this time around.