Okay let's put it simply:
He's the CAUSE of the important event, and no longer has relevance, and his story arc is played out.
He still has relevance. He's still a noteworthy character, he still has knowledge of the Veil and Source, he's still an Eternal one of the race that became "Gods" of the realm.
Bandwagon fallacy if it's being used as an argument here.
Only if you legitimately beleive that your OPINION is somehow material evidience to the fact that Fane is not a generally well liked and considered very well written character.
Which is countered by your very next sentence where you state:
Like yeah, of course, the **literal entirety** of DOS2 and BG3 are considered to be higher quality than anything else larian has made combined
Meaning you're outright stating that DOS2 (Including all of its characters) are some of Larian's highest quality work.
I am just speaking my opinion, although it is better than the opinions of people who want the 6 characters of DOS2 to return and don't realize DOS2 was a low effort cash-grab capitalizing on the success of DOS1.
Talk about logical fallacies... Your opinion is no "Better" or "Worse" than anyone elses. Opinions being purely subjective does not carry any notion of quality.
Okay, considering what you said you would think your conclusion would be "would play a massive story role," not "would show up again."
Both would likely be the case. Especially with regards to their notion of making this new Divinity game more accessible to newer fans of the studio, those that may not have played the prior titles before BG3.
Meaning that he might simply appear just so he can recap the lore for newer players. Both his part in it and the general events of DOS1 and DOS2.
But besides that;
Fane is important as an Eternal - Especially in the stated setting for the game where "The gods are silent" post-DOS2's source purge.
He is also important as a connection to the Veil. He is uniquely capable of accessing it, even among Eternals he was the one who had the most knowledge of it.
If any game has any connection to the Veil, it's likely that Fane will be around to play a major part in the story.
But you didn't point out anything I wasn't aware of already, I'm not arguing this it's just my opinion.
And yet... Here you are... Arguing...
Including disparaging other people's opinions, claiming your opinion is somehow "Better" or more noteworthy than others and trying to claim that DOS2 both sucks and is also one of the best Larian titles to date...
