TL;DR Fights where you could either choose what phase you wanted to work on, or fights where a phase 1 wipe just battle rezzed everyone and allowed them to do phase 2, 3, 4... could be a good way to reduce burnout while keeping end bosses as epic encounters.
Part of the reason I post this now is because it'd fit well for a (possible spoiler, click to see) Murozond encounter.
Ever since Kael'Thas v.1.0 we've known that fights with phases of increasing difficulty are exponentially harder than fights with easier final phases. By making these changes that could be smoothed out.
One way the game could be made more difficult in a way that would give top end players more of a challenge without shutting out more casual players would be fights with very different phases, except the phases could be practiced independently.
There's a couple of ways this could be done.
First it could just be like how the Malygos drakes worked, where there were "practice" drakes as part of a daily that you could mess around with and read the abilities of without being in Phase 3 of the actual fight.
A second way would simply be a fight where for whatever reason you could just select any phase of the fight to start on. This would probably work best for a robot or mechanical-themed boss, although 'magic' can be a catchall of course.
A third way, which I think would be cool for Murozond, is the "Grand Magistrix Elisande" way where you go back in time and do every phase. Although maybe that's a bad example, because I'm actually picturing, say, a five-phase fight, but if you wipe on phase 1 everyone just gets battle rezzed and phase 2 starts.
This would be really good because it'd prevent burnout somewhat as you're not doing phase 1 over and over and over, but if the fight is 10ish minutes long, then in an entire raid night of 3 hours with 5 minutes between wipes / breaks, a guild would get 12 pulls in. Then each night they'd only spend a max of 24 minutes doing any single phase. So it wouldn't be like that first night of Mythic Kil'Jaeden progression (and many other bosses such as Denathrius, Rygelon, Anduin, anything where one mistake = doing phase 1 again), where every time someone misses a meteor soak you do Phase 1 again and nothing else.
It'd be good for mythic because the phases could be really action packed. Kinda like Rygelon, who...I think his entire thing was scripted to take only 90 seconds, but there was so much going on and people had to DPS the adds and go to the void so dang fast that it was really quite difficult. So the cutting-edge guilds could bash their heads on 5 super hard phases and have something to do. But if the phases have to be done sequentially, then the time commitment to learn each progressive phase makes the fight exponentially harder on guilds where the members aren't at a world-first level. This would make it much easier for guilds for whom Cutting Edge isn't a foregone conclusion a chance to learn the whole fight.
But it's also a good addition to the game for more casual guilds, as they tend to spend more time taking breaks and letting dogs out and talking about strategy for 15 minutes instead of just pulling. By allowing them to see every phase every pull, they'd get to spend more time actually playing the game because their longer wipe recovery times would happen less often.
But I really do think this would be a good change that caters to players of every skill level. And WoW is such a mature game that there are enormous differences in skill levels.