The issue is more the kind of evil that makes its own plans, so that the campaign events become nothing but the backdrop to their own schemes.
The literal same issue applies to heroes.
Yet no-one has ever had concerns over heroes being shoehorned into a particular story and specific "Good" actions.
Like, Mass Effect 3's "Good" ending is wiping out the Reapers. What if someone was the kind of good where they wanted to make peace with the Reapers?
Same exact situation.
The tadpole scenario allowed for a broader range of alignments to fight towards a common goal, but with your "normal" campaign, an actively evil character would increasingly become the main villain.
Well, yes. That's literally what the "Enslave the Absolute" ending is.
It's the whole premise of Astarion's plan to do so. Become the big bad villains because you took control over the Absolute in place of the 3.
I don't get what this has to do with anything.
you'll have to create different opposition and hopefully have a resourceful player who can surprise you. It's a different campaign.
No it isn't. The campaign is still the same, with the same opposition.
The 3. The brain. That is the opposition and the campaign as a whole.
Doesn't matter what you do, evil or good (A good character can run around and ignore all the Tieflings and Goblins too). The opposition and campaign is exactly the same.
Even if you ally with one of the 3, the other 2 remain opposition (As well as the Brain itself)
That's all fine if you know it beforehand, but having to write a game without knowing what exactly an individual villain may do, is really tricky.
It's literally no different to writing for a hero.
Heroes could potentially do many things. But they're written for doing one thing.
If anything, it's EASIER to write for evil because of the very nature of free will allows someone to create a murderhobo route themselves. Like, even if none of the actual story requires it, nothing stops a player (On any route) from just outright murdering all the NPC's they come across. Meaning you get 2 types of evil route for the price of one. (Unlike good route where there's no innate "Good" action you can perform irregardless of writing, the only good actions are ones explicitly written in)
