The reason that Oghren, Sigrun, Nathaniel and Velanna aren't in DAI even if they're all still alive is that they're traveling with the Hero of Ferelden or the Orlesian Warden on the quest to find the cure. (I mean, I don't know about your Wardens, but my 'canon' archer rogue definitely needed a party with her to operate effectively.)
Frankly, I wouldn't be surprised if this particular hypothesis ends up confirmed once things become Wardenny again.
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I headcanon that my Warden and Anders were very close friends. He gave my Warden his earring as a gift. That was my headcanon and then in DA2 I noticed that Anders actually no longer had that earring. 
I headcanoned that my worldstate's Warden was deeply involved in Anders' underground railway: all those mages fleeing the Free Marches had to go somewhere safe, and Amaranthine was a perfect haven: it was ruled by a former Circle Mage who Anders and Justice trusted, the last Templars who had tried to capture a mage had ended lacking their heads.
Plus it fits elegantly with the rebel Mages fleeing to Ferelden: by the time the Mage-Templar wars started, Ferelden would already have had well established reputation as a safe haven for mages.
I also imagined that part of Anders fall into murderous madness was due to his interaction with the Warden Commander: after years trying to evade the Circles, he saw a fellow peer standing her ground and violently forcing the Templar Order to behave and imagined he could emulate her methods. Alas, lacking the all-mighty plot armor that insulated his former boss and protector from the harmful consequences of her most questionable decisions, his attempts backfired and he grew more and more desperate as time passed.
Moreover, King Alistair's visit wasn't simply about meeting Hawke, but about bringing him back home, by force if necessary, because Anders' allies in Ferelden realized that the situation was getting out of control, but Meredith got wind of it and intervened first.
To conclude with that hypothetic tale, the whole "Quest for a cure" thing wasn't simply about finding ways to counteract the Taint, but the result of my (very selfish and always prioritizing self-preservation when pushes come to shove) worldstate's Warden deciding that she'd rather be very far away when people would come knocking at her door to demand explanations about her protege's actions.
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*My Canon Hawke tried to steal a Griffin from Weissaup; not his grandest plan ever. He was caught immedately, though they let Hawke go for some reason *cough* Queen Cousland caught him and decided to be merciful towards Hawke *cough*
THAT should be made part of the actual story. Way too funny to go to waste.
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Anyway, my favorite headcanon, the one I would love to see confirmed in the actual series... is that Mabari's lifespans are much longer than ordinary dogs': the Warden's Mabari is therefore still traveling with its master, expanding the Empire of Mabaridom one tree at a time, while Hawke's Mabari is still in Kirkwall, where he serves as the de facto vice-captain of the guard.