There story IS over. Sure, here's going to be more to their life, but their main story is over: stopping the Fifth Blight, ending the civil war, and defeating the Architect and/or Mother.
BioWare has said this on several occasions.
And i'm saying we aren't gonna actually know that until the series comes to an end. the developers could change their mind at the drop of a hat or they are misleading or the Warden is done. I'm not a dev and as far as i know neither are you, so we don't actually know.
The Warden is really only interesting and truely useful as a weapon versus the blit. Other than that they are just generic warriors. I don't see that character being more interesting in any future role than any other character would be and in fact I'd think he could be less interesting in most cases.
Plus, you'd have to do some sort of ME2 type burn him down and start over or else I need my high level Warden blowing away Mages with Mana Clash from the word go.
their are reasons some want the warden back.
1. The warden has become the embodiment of everything they loved about DA:O and his return would signal the return of that aspect of the series.
2. the companions. returning to the warden (with all the choices you've made in DA:O) after playing as Hawke and the Inquisitor, there is an appeal to that. a continuation to what you've build in Origins. The relationships the warden has made, the Romance path you went down, dealing with repercussions etc.
And of course having Morrigan as a companion again, ah the utter joy. 