No matter what he would be, and i am sure ther we would have a gay/bisexual character, i hope that he will not be forced companion/advisor like Cullen and Dorian, will not have not only good but have a some bad endings(even if he will not have death endings like someones, at least something like drunk alistair) too as others, and not ignore our game decidions, especially if they was connected with him
Not every character has a bad ending: Morrigan is an example of this as is Varric. This is likely to limit the variables that would have to be dealt with if they are expected to return for a future role. Dorian seems to be set up for a future role seeing as it is heavily hinted the next game will take place in the Tevinter Imperium and Dorian always leaves for the Tevinter Imperium.
Dorian was not care about everything what we can done in Inquisition and no matter what, when we can not have him in Inquisition or banish him, or make him break relations with his father, and not find his family medalion...he is no matter what still become a altus magister and member of the Senat, and for actually doing nothing, this was make some sense if he was a member of Inquisition and really fight with Coryfeus
Dorian does care about what the Inquisition does: he outright says in his personal quest that he joined the organization because he thought it was the right thing to do so. He even wants to stay to aid the Inquisition even if he has no fond feelings of the Inquisitor. The reason he can decide to leave is if the Inquisitor cannot maintain professionalism.
Speaking of which, Dorian can leave the Inquisition if his approval is low enough since it triggers a scene where the player can select an option that results in him leaving.
Personally, I think it is a good thing that the player cannot decide whether or not Dorian forgives his father because I don't think that should be the player's decision to make. The player through the player character can encourage Dorian to finish the talk or leave. Given that Dorian is about making choices for himself, this only make sense.
Seats in the Magisterium are inheritable. This is somewhat like saying that someone has not earned the right to be a prince or a king through merit: that is not how this specific system works. You are appointed a magister by someone else and inherit the position from them with the approval of the Magisterium.
So i think they should be equal and should have consequences of our decidions as have a bad endings not only good ones cos this not fair compare to all others and ruin roleplay when you cant really disagree and become enemies of such characters as deal with them, you can only agree and be their friends/lovers or when you totally disagree with them you can only just make them go away without any bad things...they are like Tallis
Zevran was a good example, as Fenris, as Bull
As I've mentioned above, Dorian can leave due to low approval and actions chosen by the player so he is an example of consequences: if the Inquisitor chooses a certain option, he permanently leaves the Inquisition. The player is not forced into a friendly or romantic relationship with him: they can disagree and outright resort to violence with him.
TL:DR: You are factually wrong on the primary point of your post as the player character is not forced into a specific relationship with Dorian as their relationship with him comes as a result of their actions and Dorian can leave the Inquisition including as a consequence of said actions.
Note: Sebastian is an additional example of a character who does not have a bad ending.