Will or should DA4 return to the darker roots that DAO had?

This is true but the DA2 friendship/rivalry didn't conform to this because once you reach a certain point in friendship or rivalry you are locked into that relationship no matter what you do in the future. A case in point is if you sell out Fenris and give him back to Denarius. Not one companion breaks with you over this and they even have Anders giving tacit approval, which despite their differences would actually be totally against his ideology.

They actually did this better back in Baldurs Gate 2, which whilst bound by the rules of D&D and professed alignment, at least did have companions leave you or attack you if your actions went too much against their beliefs. Likewise in DAO, if you pollute the ashes Leliana will attack you and force you to kill her, even if you are her lover. (Or have I got that wrong since I've never done it?) Under the DA2 system, in order to bed her, you'd be so far along the scale one way or the other that she'd do nothing but voice her disapproval.

Having been replaying DA2 recently, I do find it hard to understand why Anders, who is so committed to mage freedom, would continue to have anything to do with a rivalled Hawke. It is not possible to get sufficient rivalry to lock him in, particularly if trying for a romance, by simply criticising his decision to merge with Justice. You have to do things that are in opposition to his professed ideology. Otherwise you are going to end up on the mid range scale and the next stage of your relationship is not going to trigger anyway. Why does he feel he owes you anything; you wanted the maps, he wanted to free Karl. Job done. Why, when he says he doesn't just go for superficial looks but how he feels about the whole person, would he even be attracted to a Hawke who actively appears to support the Circle system? Certainly, why would he trust them with the secret of the mage underground or get them involved in any way with his final plan?

Fenris I can understand because he has no one else, same for Merrill, at least in needing support, but surely there comes a point at which Aveline feels she can no longer work with Hawke even if she doesn't actively work against them. If she is in the party when Hawke hands Fenris back to Denarius, she should actively oppose this because slavery is illegal in the south and this Magister shouldn't even be able to operate in the city at all because he is a mage. Even if she doesn't want to risk her life over it, that should have been an end to any further co-operation. Actually the same is true if she is aware of how much Hawke is engaged in the mage underground, actively working against the Templars and even framing them. Turning a blind eye is one thing but still working with them?

The friendship/rivalry system sounded good in principle but it had a lot of holes. I'd rather just a straight forward system where if you do something a companion disagrees with, if it is not of major significance perhaps on that occasion they let it go but enough instances of this and they will eventually want out (I think this can happen with companions in DAO like Zevran or Sten). If you do something that is ideologically contrary to what the companion believes in, at the very least they want nothing more to do with you.

I don't think they have it in them anymore to do a DA Origins type of dark, as much as I'd love to see it. Safe and sanitized seems to be the way they want to go.

I hope that this won't be the case. I've mentioned before that the one thing I really want from DA4 is meaningful choices, and in Tevinter the most fitting way of accomplishing that is to simply determine who bears the consequences of your actions. A hero shoulders burdens so that others don't have to, but what if they prefer that the negative consequences of their actions fall on strangers, slaves, or foreigners? It's morally reprehensible, but it's also easy, and in Tevinter of all places we should have the option of taking power and shifting responsibility, it's been their MO for centuries. That would darken the tone not simply because of what you encounter but what you do. And how far you're willing to go to accomplish your goals, be they virtuous or selfish.

We need the opportunity to be Branka feeding her people to the Anvil of the Void, or Celene Valmont declaring “Sitting on my throne, I see every city in the empire. If I must burn one to save the rest, I will weep, but I will light the torch!” Without the temptation to do wrong for the sake of convenience, which was largely missing from DAI, where is the nobility in choosing to do good?

Celene's line should actually be "If I have to burn a city to soothe my own butthurt, then I'll do it and never ever feel bad about it."

And that really sums up Dragon Age to me. It's not remotely morally ambiguous in reality, but stupid and self-serving people will say that it is, because they want an excuse for living out their worst, most bigoted impulses.