First of all, people need to understand this: Bioware can only do two things with the Warden Commander: 1) Bring him/her back. 2)Mysteriously kill him/her regardless of our choices. Which one is more logical do you think? Because at the end of the Awakening, The Warden Commander lives no matter what, so just forgetting about such an important figure would be ridicoulus. The Warden Commander revealed the truth about how the Fift Blight began, met the Architect, met Anders and Justice (both are obviously important for the story) and if the Warden Commander was also the HoF, s/he also defeated the Archdemon and even Flemeth herself. If even Hawke was brought back just because s/he fought Corypheus so s/he could help, then why wouldn't the Warden Commander come back when the Architect or another Archdemon appears again? Especially when a Warden with great influence like him could help to resolve the matters in Weisshaupt.
Also, it's only natural that people expect to see more of the Warden Commander in the future after seeing DAO Awakening Epilogues. In the end of the epilogues threre writes stuff like these: "neither does anyone think her tale is complete ..." , "neither does anyone think his tale is complete ..." "simply disappeared. Most assume their tale is far from complete ..." These lines appear for both HoF and Orlesian Warden Commander.
Also, if the Warden's relevance to the story simply ends with the first game, then Hawke's relevance has never even truly began. The Warden is obviously much more important than Hawke. If the Hero of Ferelden is alive in DAI, Cassandra says that "First, Leliana and I searced for the Hero of Ferelden but he had vanished. Then we looked for Hawke...". He was the Inquisition's first choice. He is DA fans' first choice as well, clearly. Ultimate Party results were a huge crushing victory for the Warden. Simply forgetting about him/her wouldn't be the wisest thing Bioware can do, that much is obvious. Especially after even Hawke was brought back.