You know, I really wouldn't mind if the "best" endings were only achievable in the higher difficulties. 
I'm sure if there was a dislike function this post would get like a 100 dislikes. 
In all seriousness, I don't hate sliders but I definitely would like it if there were actual in game rewards to playing on the higher levels (be it with item rewards or story outcomes).
Anyway, I doubt DAI will be like this at all, but it's a nice thought.
It'd be nice and you're probably right about the dislikes. I'm still used to the best ending requiring the defeat of an optional boss that requires a very solid battle plan, high level and top end equipment. For me there needs to be an incentive to raise the difficulty. Take Mass Effect 2. I raise the difficulty to obtain the geth pulse rifle. Now having the difficulty influence the story that would be wonderful to me in part because it'd feel like my character is defeating storyline level enemies. I've often wondered at what setting it'd be safe to say your character is fighting opponents with storyline level of ability.
About the possibility of a better outcome depending on our performance in combat, I'm totally against the idea. I'm all for rewards for those who excell at combat, but those should be also combat related. I could agree that it would be coherent, from a certain point of view, but I think players satisfaction is much more important, and not everyone cares that much about that part of the game, or has the ability. I have never been remotely interested in playing DAO or DA2 in nightmare, and I honeslty doubt that it would change with DAI (though it would be great if that was the case), so I'd feel terribly and unfairly punished.
Not everyone cares about exploration or having to pick a choice they find disagreeable just to get the outcome they desire. I personally really dislike having my character's combat ability ignored. I find great satisfaction in having my character's abilities actually influencing the story. I'm reaching a bit back here but let's take Vanguard Bandits for an example. A nice real time strategy game. There is an arena fight and your army actually responds to your performance. If you lose early they lose loyalty because they trust in your ability less but if you not only win but beat the day lights out of the arena master despite being fatigued from battle they get a huge boost. This is fairly normal through the game the more of them you keep alive through each battle the more they trust you to keep them alive and get them home. If you do a really good job they're a bit on the fanatical side near the end and why shouldn't they be? You've guided them to victory after victory with few losses against some rather powerful foes. Complete victory also inspires the arena master to not only give you more soldiers but also to join your army which is awesome as he is a fine heavy unit.
You mention Mass Effect 2. It rather annoys me that certain squad mates die in the pipes no matter how fast you make it. Heck I've actually been running ahead of my squad mate before opening tubes as I went. How does it make sense that they didn't have time enough or that my Shepard couldn't protect that door? The average survival rate for enemies in her presence was a matter of seconds and many die before they can hit the ground.
Thinking on Inquisition. What soldier wouldn't be inspired after seeing their commander sneak into an enemy fortification with three lieutenants and open the door revealing they've already kill everyone inside that so much as thought of holding a sword?
Now if the developer doesn't want these kind of super human acts to be part of the story they shouldn't let the character develop that kind of combat strength. Tone it down.
We all like what we like though and it's natural to want those aspects of a game you dislike devalued so you can succeed. I still think of role playing games as a mix of many elements though and believe in embracing all of them to get the best outcome. Having dialogue choice dominate is no more agreeable to me then having combat completely dominate. It'd be nice if some choices carried long term penalties to. Growlanser heritage of war I believe or perhaps sense of justice have a nice moment with that. One of your allies needs to get back to the capital city but the main character has been poisoned and the toxin is a new strain that can't be cured with the medicine you have on hand.
You have to choose to try to make it back while poisoned or wrest. Your poisoned throughout the event and that makes combat much tougher. Which I loved. That said if you can make it back that companion is far more loyal though she now thinks your absolutely crazy for demanding a forced march in your condition.
It could be such fun if the Inquisitor had to set up a quick defense and you had to decide how many of your health potions to give up. If you choose all everyone is moderately stocked on potions but well you have none. Perhaps the Inquisitor can even refrain from telling the people that she/he gave them all she had. Now begin the epic battle. Unfortunately this scenario loses any bite if you can just turn the difficulty way down so that nothing can even scratch your character's armor.
I wonder how those who don't enjoy combat but want dialogue choices to matter would respond if those who enjoy combat demanded a slider that would make all the Inquisitors dialogue choices automatically get the best outcome.