overpowered stuff like wish spells and such should probably stay off the table, because otherwise one could argue you can solve the whole plot with one. Would be a plot hole. And Gale's scroll hardly works narration-wise - where's the tension if the solution is walking next to you for the whole game?
Gortash options make a typical "is revenge really worth it?" storyline, which definitely could be a nice angle! But we'd still need his Netherstone for the plot, which is a problem. Unless I guess we begrudgingly ally with him in this scenario and it goes from there as such. (Doesn't he die anyway once you reach the brain? I think he does.)
(although him being a key to salvation would remove Karlach's monologue after defeating him - and I do love that monologue :") )
Scheming with Raphael sounds like fun, not gonna lie. Although Karlach would definitely disapprove - but that could be the intentional catch, you know. Plot-wise it's actually baffling (I mean, in-game-world) that he talks about losing hope and desperation, and seems to be a tad obsessed with Tav in general, but never once he thinks to pull the string Tav who's romancing Karlach might be ACTUALLY desperate about.
In all honesty though, the Gondians and the infernal iron of like three or four varieties, however, are the WORST offenders here. So much so that it's hard to believe that all those threads WEREN'T put there for a reason originally. Like, COME ON.
I believe there's a way to fit those endings in the setting, but this ain't it, and also the fact that they are the ONLY endings possible - now that doesn't fit. You can screw up beyond repair in D&D, sure, but just saying 'eh, no options' and throwing your hands up in the air? In D&D?! Yeah nah.