BioWare could easily work the Hero of Ferelden back in....here is how.
SPOILERS?
Plot Points: Flemeth --> Urthemiel (Fifth Blight Dragon) --> Warden
Just because a body is destroyed doesn't mean a being is dead as is shown in the case of powerful beings like Flemeth, Corypheus, the Dragons, etc. and certainly the Warden could fall under this category.
BioWare never explains what happens when the Old God soul and the Warden's collide...perhaps they merge as one or fight in the Fade, like how Kieran and Urthemiel inhabit one body...or Mythal and Flemeth.
So Origins has to end in a certain way:
Warden dies taking Urthemiel.
Loghain dies taking Urthemiel.
Alistair dies taking Urthemiel.
Kieran (Morrigan's son) gets Urthemiel's soul.
In Inquisition, we see Flemeth either take Urthemiel soul from Kieran or take Kieran himself. She has clearly been after the Old God's soul since Origins.
The Warden can be worked in like this: (Maybe DLC!)
1.) Going after Flemeth to reclaim the soul.
2.) Flemeth summons from the Fade the Urthemiel soul fused/connected with Warden/Alistair/ or Loghain's soul.
*in essence this could bring the Warden 'back to life' or actually back from the Fade (quite possibly with an Old God's power for him/herself.
And...I for one would love that. Origins was a great game and many other Origins characters play a huge role in the direction of Thedas....and Warden should too!