Hmm. DAO had political in-fighting among the humans (and dwarves). Remember Harrowmont vs. Bhelen? And Loghain vs. Cailan? And Loghain vs. Eamon? And Loghain vs.
everybody?
DAO also had some grey-and-grey moral dilemmas ... e.g. "Do I let the Pride Demon enslave the templar, because she gives him the happiness he never knew? Or do I kill her, because - duh! - she's a friggin' demon!"
In contrast, I don't remember that kind of dilemma in DA2 and DAI.
In DAI, particularly, it was a case of "Hey, lookit! It's that character from DAO again!"
That happened with Morrigan and Cullen and Varric and Dagna and Leliana and ..... you get the picture.
DA2 had that, too, but only with Alistair and only for one scene.
I like you idea of a new kind of threat. An evolved Darkspawn, perhaps? Maybe take the idea of the Architect (from DAO: Awakening) further - maybe now that they can talk, the Darkspawn form their own civilisation. A crude and violent one, sure, but one that needs dealing with. How? Well, that's up to Bioware, hmm?
Besides, even if the Grey Wardens only have one job - fighting Darkspawn - that doesn't matter too much, surely? Everyone (almost) agrees that DAO was the best instalment in the series. What's wrong with creating another?