I've already shared my thoughts on my personal experience with the endings on a previous thread (https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=944544#Post944544), but I wanted to share my suggestions as to how the ending could have been improved in key areas. As of now I'm not sure Larian would update the ending any further, both because the game as a whole is in a good place now and further patches will probably cause issues with possible modding. Therefore this post could be taken as a framework for how Larian could improve future games' endings, though in the slightest chance of a definitive edition, something similar to these suggestions could be implemented. Or: TL;DR, a third option/perfect ending should be possible under the right conditions, but it should be HARD to get. For this thread, Orgon=Tav.
First off I was overall happy with the ending I finally got, but the final sequence starting with freeing/killing Orpheus is where I felt my player and character agency was suddenly restricted compared to nearly everything else in the game until that point. I don't recall a single persuasion check I could use against the Emperor in the whole game, and it wasn't until the end that I was able to convince Orpheus to keep living a bit longer, long after I could have used persuasion/charisma for greater impact.

Orpheus vs. Emperor "Third" option
So the first suggestion has probably been suggested many times by others; there could have been an option to convince the Emperor to free Orpheus instead of harvesting his brain. It should be difficult, granted, but its difficulty should be affected by how you treated the Emperor thus far in the playthrough. I'm not sure how this could be measured, but it is a complaint I have heard and isn't without merit. I imagine though that this should be impossible if you were rude to him enough. The logic to use behind it could have been "If you kill Orpheus now, you will be hunted down by his followers." If Lae'zel is present, she can back up this argument. Then when the Emperor asks how you could possibly guarantee that Orpheus won't immediately turn on you both, you could say something like "If he does, I'll restrain him long enough for you to harvest his brain." Then the Emperor could retreat to a safe distance while you deal with Orpheus.
I'm fairly confident that the conversation you have to get Orpheus to help you would go much the same. I would have liked him to not accuse me of stealing the githyanki egg (I mean, how did he know this, and if so, why does he not know I still have it, and Lae'zel is protecting it? Not to mention it was going to be destroyed without intervention) as a slight against me. I think that if you tried to spare his guards by knocking them out, but they inevitably get killed by the Emperor later, that would affect his perception of you and make the persuasion check easier to get Orpheus to then work with the Emperor against the Netherbrain. Though I think the factor that should matter the most is how many tadpoles you had used up until this point; using the Astral tadpole might effectively lock you out of this option with a 99 check, etc.
The rest of the section would have both the Emperor and Orpheus tagging along with you against the enemies leading up to the Netherbrain. This would all be largely the same, with the Emperor the one being the one to channel the crown, and Orpheus using his invulnerability globe on him from a gameplay perspective.
It would be at the docks once the Netherbrain is dead (assuming you chose this) when the two would finally need to resolve their differences. Orpheus fully intends to kill the Emperor for being a mind flayer as well as having him imprisoned for that long. The Emperor does not want to fight and would rather be left alone. You could step in and side with one or the other. If you convince Orpheus to stand down, he will let the Emperor go on his word that the mind flayer will not interfere with the Githyanki's quest for freedom, which the Emperor would truthfully agree to, and leave. However, my character would have wanted to kill the Emperor as well, in which would start a combat section similar to the Dwarf assassin of Bhaal: to kill the Emperor before he summons a portal and leaves. I imagine the challenge here would be that most spell slots/abilities would have been spent against the Netherbrain, so this wouldn't necessarily be easy.
If you fail to convince Orpheus, you could step in to protect the Emperor, or just let them fight. I imagine that Orpheus would surrender if he got brought down to a certain health like Ethel, though Lae'zel would likely turn on you if that option was chosen. The Emperor would escape with this option, regardless.
The ending would then play out the same as before, though Orpheus will leave for his war against Vlaakith, this time in githyanki form.

If the Orpheus route is chosen without the Emperor (for whatever reason)
In my playthrough, the point where I felt a "what the hell, DM?" moment was when Orpheus said that we needed a mind flayer, and our only option was to have someone transform. When Orgon said "there must be another way" Orpheus (or DM) just rejects it outright with "there isn't". I mean, if you stop to think, there were a couple of other possible options. The most obvious is the mind flayer that I actually liked and thought of as a friend: Omeluum.
Since the whole mechanic around a mind flayer being needed wasn't well explained, I might be remembering things incorrectly about how ceremorphosis and the tadpoles work. But after this choice in the base game, we quickly teleport to Baldur's Gate's upper city thanks to Orpheus. At this point, Orgon had already saved Omeluum and talked to him/it at the Society of Brilliance: why couldn't I ask Orpheus to take us to him? There should be a persuade check though, as Orpheus would say "How do you know this mind flayer can be trusted?" Again, this should be affected by some of your previous interactions with the githyanki.
A possible cost to doing this could be that it takes more time, and Baldur's gate could have ended up a bit worse off. Of course if Omeluum isn't saved, and you don't know where he is, then this option should not be available.
When you both find Omeluum, he'll likely say something along the lines of "I'm willing to do this, but I am simply not powerful enough of a mind flayer for this task." Which brings up the other possibility that would be needed for this to work; the tadpoles.
I was somewhat irritated from a gameplay perspective that the only thing you could do with the tadpoles was use them, even if from a roleplay perspective your character would use as little or none of them as possible. This is where they could be useful; if you have enough of them, still have the astral tadpole, or both, Omeluum could absorb them and be strong enough to channel the crown like you, Orpheus or the Emperor could be.
After this point, Orpheus and Omeluum would tag along with you for the final fight, though there would be little need for a confrontation at the docks between Orpheus and Omeluum provided you used a persasion check about Omeluum's trustworthiness.

The manflayer ending could have been the best ending (at least for Orgon)
I detailed in my previous post how sad the manflayer ending was for Orgon as he killed himself and sacrificing basically everything. If this had been the only choice, my opinion of it would have been radically different. The ending itself just seems... unfinished. Withers implies that Orgon's journey wasn't over, but then... nothing? As far as I can tell there's going to be no DLC for BG3, and I don't think Larian is going to update the ending further.
But it's this ending that still makes me hope they do, at least a little bit. The problem I had with this if it was final wasn't that it was sad or tragic, but rather that there was no closure with Orgon's companions. The first thing I would have liked is a final dialogue and goodbye with your party before the manflayer kills himself, and I imagine there would be quite an emotional performance from your romance option if you had one, as well as any that have high approval.
Then with the character's soul still existing despite Wither's surprise at finding a mind flayer soul, and with all of Wither's powers of resurrection and souls, surely he could have helped? I'm not too sure of the lore being true resurrection, raise dead, and soul destruction, etc, but there's enough I've read of D&D lore that such a thing should be possible.
Well, provided some conditions are met. If you never had the conversation with Withers about mind flayers having souls, and did not say that mind flayers can have souls, then your soul would probably fade away. And if you used too many tadpoles, your soul would have been fragmented enough that nothing can be done.
However, with Wither's help, it should be possible for your soul to come back, provided the previous conditions are met. A few mental checks (Intelligence, Wisdom or Charisma, and very difficult ones) should reform your soul gradually into the person you were before turning into a mind flayer. With enough successes, Withers can successfully pull you out and resurrect you. Enough failures though, and you can only come back as a soul - but still, that would have been something.
Then instead of the reunion party as it is, it would have had the party sans you gathered under Wither's guidance, and then you could have been reunited with the party; either fully alive, or in spirit form.
I imagine this would take a lot in terms of resources for a believable performance from the voice actors and animators. Assuming your character had a positive relationship with most of them, I could imagine a lot of tears being shed.
The reunion party could then proceed like it was, but the dialogue will be very different. If that was the ending I had gotten, I think that would have been the perfect ending. Though for this to work, Karlach would have needed to go with Wyll to Avernus without your input. Without everyone reunited, it would feel hollow.

Conclusion
I'm mostly unloading the ideas from my brain, and I'm fairly certain the ending for this great game will be left as it is. I was already fortunate that Karlach's additional ending was added in a patch before I played, and while the endings I speculate about here would have been incredible, the ending I got was still good. Still, I can understand why there are complaints about the ending; a different character with different views and motivations might have felt completely shut down by the ending.
If you read this far, thanks for reading. I hope Larian can continue to improve their endings in the future, as I believe, despite my gripes, that the ending was a marked improvement from DOS2 (though perhaps my experience with that ending was in the minority).