Well, the OP is surprisingly (to me anyway) an argument for "being more like DAO" that I won't get behind- not because I'm diametrically opposed but because they're not at all the aspects of DAO that shine. And such a long OP to fail too. I couldn't care less that they didn't bring brothels and rape and demons with boobs and such into DAI. I'm not happy they're gone either- more indifferent: I never intentionally partook (OK, once I did the awful Pearl sex scene- meh- but only because I'm a content junkie). The only "dark" (sexual) omission that I miss is the mostly nudity upon stripping a character- and not because the body models were so hot (though DA2, mm mm), but because- really? When I take off my clothes I have clothes on? But again this doesn't even flicker in the "give a damn" zone for me.
And as for the "dark" (horror) aspects, DAI in no way skimps. Death is a bit overdone really, often begging the question of how many people are even left alive in the world for there to be so many dead. Just look at the JoH west hillsides regarding Scout Harding's friend who torched Hakkons (and who you can then recruit to the Inquisition to "keep up the good work")- or the scene in the Western Approach where you find a journal among skeletons describing an expedition's turn to cannibalism, or any number bodies mangled or partially eaten through one's travels, or the total darkness you have to venture into despite hearing hostiles about, or people being harvested into red lyrium... And the creature models are often repulsive too- those fear demons truly quite dreadful. Meanwhile in DAO the orphanage scene was laughable in its canned sinister "eeriness." (I seriously wondered if it was intended as a parody of horror films.) It's true that nothing in the DA series can compare with the horror of the Broodmother sequence, but there's no lack in DAI. Irrelevant criticism.
And as for "dark" (evil) choices, I just don't find myself empathizing with the OP's explicit wish for yet another clan of elves to slaughter. Really? Miss that, do you? The elf-hatred built into DA is one of the most disappointing aspects of the series. I mean, no one has to put elves up on an immaculate pedestal to portray them well, but constantly disgracing and murdering them... starts to beg the question of what the devs have against them. On other grounds, I only tainted the Urn of Sacred Ashes once (then reloaded), only made Alistair a drunk once (never had him executed), only failed to save Owen's daughter once (coincidentally during my archer playthrough), etc. There's no thrill for me to play "evil," and I've never quite grasped why people say there is one. Playing controversial, sure, even self-serving at times, but "evil" tends to just make me stop enjoying and empathizing with my own character. DA2 added a better element of playing a scene sarcastically or rudely that I enjoyed a lot more than if I could please just humiliate, cripple, or kill another elf. Pretty please?
The sex, horror, and depravity in DAO were aspects of the game that were nowhere near worthy of accolades from me. Their apparent sacrifice in DAI goes almost unnoticed by me. But yet I agree that they need to get back to DAO's strengths. By contrast I'd list:
1. The origins.
OK, so it's in the title- DA:Origins- that DAO had them. But as a game dynamic that was a tremendous feature with which to immerse the player directly into lore. It was barely added to the single origin of DA2 and only half-arsed into DAI with a single paragraph pre-CC. I would say that separate origins to play through is the strongest exclusively-DA feature that the devs have to work with in the series, but they've all but completely dropped it from DA development since DAO. There is no better way to introduce Thedosian world concepts than with a range of characters that are dyed-in-the-wool of the backstory. I like reading what the writers concoct in books and notes, but codexes just don't compare to living it.
2. The origins.
Wait, said that already.
2. The rich narrative.
Loghain- whatever one may think of the character- engendered some vehement discussion on the forums. As did the Anvil choice, the selection between Harrowmont and Bhelen, the saving of Connor, etc. These weren't entirely "dark" (evil) decisions, just difficult ones. The forums for DAO were always chock full of people truly struggling with the ethics involved (or just adding a somewhat more sophisticated trolling). By contrast you mostly get gameplay debates in DAI's forums, struggling over how DAI was made... this thread, for instance...
3. Wardens.
They seem to have been tucked into a closet like an oops or an embarrassment. They're a fantastic type of "unlikely hero" to work with that has a unique and elaborate backstory, but it's like the devs are constantly looking for ways to give Loghain's Warden-hatred some more grist for the mill. Yet we played a Warden in DAO, saved at least Ferelden as a Warden, made the whole griffon-riding image complete. It would be great if one of the origins in DA4 was a Warden as well with a unique storyline to offer. Make it one of the human origins and I'd eagerly play a DA human for once.
4. Combat animations.
The expressive DW dagger lashes and the riding a dragon's head are all I can think of- but those were my favorite and one of the highlights of my overly-extensive screenshot collection. DAI does well at mage and warrior animation otherwise, other than some overdone warrior effects that look like magic spells.
5. Cityscapes.
Orzammar and Denerim may have seemed ridiculously small compared to, say, Baldur's Gate or even Kirkwall, but as to providing a window into the game setting they were excellent- enough to fill the imagination. I don't mind head-cannoning the "actual" size of a city if events in that limited section of the city feel larger. But you still need to add space. Even little Orzammar had a number of sections and eventful walks to take, encounters to find. Val Royeaux by contrast was absurdly small and uneventful without comparing it to anything, and this despite the huge wilderness regions outside the city replete with hours of colorfully empty exploration.
6. Assassination attempts.
One great BG element that DAO reintroduced was running into people who had been contracted to kill you. They end up being a motley crew of characters just like your own group, but they think they can take you out. It's even the very way you meet Zevran. DAI had none of that other than predictable "ambushes" and gratuitous respawns of hostiles.
I could list a lot more, but you get the point... DAO was great for reasons different than smut, gore, and including options to be a scumbag- and emphasizing those aspects deemphasizes the genuine quality of others.