The point about healing various sizes of party is a good one. One way to make healing more interactive and involving would be to have more of a focus on triage. Right now AoE healing is powerful, abundant and cheap. So much so that even fights like E12S pt 1 are quickly forgotten even though they threw huge amounts of AoE damage at the raid over the duration. Making AoE heals powerful but on much longer cooldowns whilst forcing us to have to prioritise who we heal is one avenue that could be explored. It's this process that made healing vanilla WoW PvP so engrossing for me IMO.
Onto the last point, I really don't think SE needs to drop the trinity system, but a rethink almost to that kind of scale does need to happen if these thick cobwebs are going to be shaken off. A heavier emphasis on hybridisation and buff abilities is a great way to further engage healers. Again, Warhammer Online's Warrior priest is probably the best example of this at least out of any MMO I've played.