Cullen's role in DAI is "military adviser". Can you honestly say that they couldn't have easily developed a new character to fill that role? Its not like Cullen really has that much established backstory for the writers to work from. What does he bring to the role that another character could not, other than a horde of fangirls? Leliana on the other hand has a lot more backstory and is a much more developed character, so its unsurprising they would continue to use her in a major role. The decision to put Cullen in that role reeks of catering to a particular fanbase.
Sure they could have made another character. But with Cullrn, you already have a character that has seen the best and worst in mages in two different Circles, was there to witness the events that led to the beginning of the Mage/Templar war, has likely the past few years fighting in said war, and has connections to both the HoF and Hawke.
With a character whose back story already has everything needed for a military advisor at this point in Thedas' history, why would Bioware bother goin through the trouble of creating and entirely new NPC when Cullen is a character they have readily at hand?
There's no need for an entirely new cast I characters when those from other games are seen rising to prominence and can fill the roles needed. It's not fan service, it's character progression. Bioware tends to acknowledge that characters evolve overtime and let us see that even with NPC's.