If you work with Morrigan or do the 15hour 'mission' through Lelinana,
you get a glimpse of what happened to the warden
He (yes, I played the demon baby warden) is in search for a way to ward off the calling.
And possibly working for more.
What I think is that he is a similar figure to 'Revan' in KOTOR 1. Even flemmeth understands him to be some what of a recognizable character.
SO my theory is that he 'knows' or 'suspects' that there is something higher at stake.
Instead of fighting with the inquisition, he wants to extend his life and fight with the 'real' threat (that being flemmeth or Solas.)
If this theory comes true, he will take part in later DLCs or in the latter instalment of the DA series. It makes sense... and I think
that would make more sense because the hero of ferelden is possibly the eldest, and the wisest amongst the heros ( inquisitor/hawke/the warden)
That's how I pictured it anyways.
But I do agree it kinda sucks not being able to see him.