I wanted MMORPGs to experiment more with solo sequences in operations, so I was happy when I discovered a solo experience was available in EV.
Infernal Council was interesting when I first saw it, but I honestly felt that it was a bit of a let down when I actually did it. The enemies are so easy, even on HM and NiM, that it's pretty much just free loot. I wouldn't been much happier if, instead of free all 8 council members at once, it freed 3 at once that you were expected to fight as a group. Whichever 3 were freed could either be determined (such as needing someone at the console to press one of 8 buttons to free them) or random (so that, when you click the console and start the event, it just randomly picks 3 and frees them). As each council member dies, you would either need to free another one (the death of one weakens the barrier for them all so you either choose one of them to free or they all break free at once; or the death of one strengthens the other 2 and you have to break the shield to hit the remaining 2 with a stasis beam that removes the buff) or a one would break free on its own (i.e. the death of one breaks the shield on another; could either be random or programmed such that each one has an order of target that they unlock or it unlocks the closest imprisoned council member when they die).
Everyone fighting their own target seems like it would be epic, but, in a game where the holy trinity of roles exists, it's just kind of a let down, especially since it's not really apt to force people into challenging objectives outside of their chosen role (e.g. healers often heal because they don't like DPS, not to mention how hard that fight makes it on positional DPS). However, it *would* be interesting and apt to see a piece of content where you free pairs of bosses, one of which is allied with you (I imagine it being one Jedi and one Sith in each pair, their allegiance swapping based upon which faction you are for the raid but each boss being functionally identical), that are in eternal stalemate and need the help of a specific role in order to win: for tanks, you have to actually taunt the enemy boss and survive it while the ally boss heals you and does some damage; healers have to heal their boss through a lot of damage, forcing them to put out a certain minimum amount of heals/sec; and DPS have to beat down and interrupt their enemy boss before their ally succumbs to his wounds which can't be healed thanks to a trauma-type debuff. Each solo "boss" would require the explicit functionality of the chosen role while still maintaining solo performance requirements and feeling suitably epic (you're coming to the aid of an amazingly powerful ur-Jedi or ur-Sith and finally ending the eons old battle they've been carrying on. It would be like breaking Captain America and Red Skull out of their imprisonment just to help Cappy beat the crap out of Red Skull in their final dramatic confrontation.
Instead of requiring everyone to succeed, however, I think it'd be more interesting to design for the possibility that some players will fail in their tasks, either by dying (temporarily, in terms of the operation, of course) or by failing to accomplish some goal that will make a future encounter more difficult (but not impossible). Imagine, for instance, failing to kill your target for the Infernal Council. Because of that failure, you either aren't available to kill Soa, or that Council member will become a part of the Soa encounter (and much more powerful, for whatever reason you care to make up). Just an example.
The problem I can see with that is that is sounds more like a single continuous fight rather than 2 distinctly different boss fights. Using your example, the Infernal Council would be phase 1 of Soa wherein, if someone failed to kill their council member before Soa freed himself (the timer on his stasis break would need to be lengthened), you would have to deal with said Council member who has been suped up by Soa's aura/battle meditation whatever (so that he's a threat to the entire raid rather than just the one player) while taking care of phase 1 Soa. Honestly, I think a fight like that would be quite kewl, especially if the solo fights were actually challenging enough that only the best groups can manage to get through p1 with all of their council members dead and most have to deal with a 1-2 council member p2, but it wouldn't work as separate bosses.
As separate boss fights, you would have to deal with loot distribution and phase resetting/progress saving. For the first, it's relatively simple: if the fight ends regardless of whether you successfully killed your target or not, what's the point? The loot you get just ends up being given out for attendance rather than even some modicum of performance. It could work for a miniboss, wherein the rewards are not explicitly raid loot (such as the miniboss providing stabilizers rather than actual loot), and it serves as a precursor to the actual boss fight. A possible workaround would be requiring a certain amount of progress on the boss before a certain time limit (such as the boss casting a giant nuke that kills everyone after time is up if they're above 25% health and fleeing to join with Soa if they're below 25%, with them not being able to flee and join Soa if they're dead by that time), but you still arrive at the second problem.
If the quality of how you cleared one boss affects how a future boss behaves, how do you deal with it saving said clear and how do you deal with conflicting saves if it does save said clear? If it's saved as a buff applied to the boss in the given phase, players could just reset the phase while preserving their boss kill and not having to bother with the "harder" difficulty. If the boss kills *remove* a buff that the boss has naturally, when the phase resets naturally (such as when a raid group calls it a night and comes back later or there are some phase access problems and it has to be bounced around an reset to let someone in, which happens often enough to be an issue), you're forcing people that did a high quality clear to do the boss at the same difficulty as those that did it with a low quality clear unless you want to force them to repeat their prior performance, only this time without actually getting a reward for it (which could get very annoying if the content is actually challenging, not to mention that it gives groups a chance to just keep trying over and over again until they get a "good" clear of the boss in order to prep for the final boss). If it is saved to their weekly progression on their character, what about conflicts between characters? If one person has a perfect clear and another has a mediocre one, does everyone else get to ride the coattails of the perfect clear even though they had mediocre performance or does the perfect clear get penalized for running with someone with a mediocre clear by having it default to the worst performance or would it just use the leader's clear, which would mean that the person with the best clear becomes leader and starts the phase and the leader spot gets passed back, which amounts to the same thing as the first?
It's a Pandora's box of problems.