As think I introduced Cullen into the thread--which I now regret thanks to your apparent poor opinion of him--I feel compelled to step back in in support against your assertion that he was present only because there would have been backlash otherwise and the devs didn't want to deal with that. Of course, I may be biased because he has always been my favorite character of the franchise and I cannot know the minds of the devs and the writers, but it seems foolish to me to compromise the integrity of a story just to cowtow to a supposed popular opinion because how exactly are you supposed to write yourself out of that hole later?
Where you seem to see unnecessary fan service, I see opportunities for character growth which serve to enrich the narrative.
If we look at the default worldstate (which, as I understand it, is not supposed to be BW's canon but is primarily to reduce the amount of referencing to past titles), there is a Female US Mahariel Warden. This means that if you started with DA2, you would have gotten those conversations about Mahariel from Marethari, Merrill, and whoever else in the Sabrae clan felt like talking about them. You wouldn't necessarily know Pol from anyone, but he's there too.
Does that mean that the inclusion of the Sabrae clan was fan service? Or that Merrill being a companion was fan service? I would have to disagree with you if that is your opinion. We would not have been granted a few more tidbits of Dalish lore without the Sabrae clan's forced extended stay in the Kirkwall area or without Merrill's companionship. The Flemeth hook would not have been present and, while her importance was not immediately apparent, that is what sequels are for!
Additionally, neither Hawke being from Lothering nor serving at Ostagar if not a mage are details I would consider to be fluff (aka 'unnecessary'), but rather additional knowledge that serves to ground the new protagonist in the familiar. Loghain and Ostagar have come up in conversation for every game because they are hotbutton topics for Fereldans even 10+ years later.
To give a DAI example of this, I'll go back to Cullen. Regardless of whether or not you play the Mage Origin, a DAO player will always encounter him during the mandatory 'Broken Circle' quest. If we ignore that and skip to DA2, he was one of the characters who benefited from tossing out the epilogue cards as he was sort of 'evened out' as a result (no snapping and slaughtering apprentices or succeeding Greagoir with his anti-mage sentiments intact). But Mahariel sides with the mages, so he is always denied the certainty of purging the Tower (so iirc that would be a Cullen that would've snapped prior to the cards being thrown out).
I remember that I couldn't get to the Wounded Coast fast enough after one of the recruits mentioned 'Knight-Captain Cullen.' I couldn't believe it could possibly be the same person. I thought I must have heard her wrong or that Cullen was just a common first name--I assumed Cullen had still been sent to Greenfell to 'level out' and was then reassigned to a small town Chantry somewhere elsewhere in Ferelden, never to be heard from again--imagine my delight to see him there...even if he was being a bit of an idiot, running off after Wilmod alone. But he showed a growth and maturity throughout the game regarding his opinion on mages (my opinion! If you'd like to think he was a pitchfork-brandishing rabid mage-hunter that is your right and only tangentially related to the discussion of 'fan service' which I am attempting not to stray terribly far from).
I was over the moon to hear he would return in DAI because I am so used to the characters I appreciate the most being completely disregarded in future installments of things that I felt like such a lucky duck to even know what he was doing...let alone be able to interact with him with any sort of regularity. I expected something similar to what we got with Aveline i.e. confirmation of his status as Kirkwall's Knight-Commander during their recovery at best prior to his reveal). And again I felt as though Cullen was a necessary/logical bridge to the past. His experiences with the cruelty and barbarism of both sides of the Mage-Templar War make his perspective invaluable to any Inquisitor who wishes to take advantage of it. And I wish more attention would have been paid to that particular conflict instead of what seemed to amount to a game of 'Hungry Hungry Hippo' Quizzie vs Corypheus style but considering it was not the main focus of the game, that's a bit unreasonable of me.
Could they have developed a brand new character to serve as the Inquisition's Commander? Yes, I suppose they could have as they did so with the Ambassador...but I felt as though Josephine's credibility was enhanced by the conversations I'd previously had with Leliana and Zevran back in DAO because I knew more about how ruthless--and deadly--it can be to successfully navigate the politics of both Antiva and Orlais (again, mentioning Antivan Crows or Orlesian social norms do not fit the proposed definition of 'fan service'). So even without Leliana's personal endorsement, I felt she was qualified and respected her experience. My Inquisitors don't pick their advisors so it's a moot point what their opinion of her qualifications are. However, I still think it would have been important to pull from a respected military force such as the Chevaliers or the Templars if only for that person's posting to be considered more-or-less beyond reproach.