I love Sigrun too, but there is a few of issues with her return:
1) If the player left her at the Keep, she's dead.
2) Kind person or not, Sigrun was a Death Seeker.* Even in the sessions where she lived, the Warden-Commander had to keep her busy so Sigrun would run off to the Deep Roads to die. The Warden disappeared, so those distractions are gone.
3) She could have very well had her Calling already. The "30 year" rule is something David jossed awhile back, stating that some Wardens live longer than others, but the max a Warden can live post Calling is 30 years. According to him, some even go on their Calling after 10-15 years.
And it's been ~13 or so years since Awakening. Doesn't look good for her. 
*Edit: She's even got her own entry on the page:
Sigrun is a lot more cheerful about it than Zevran or Sten, but still makes it quite clear that as a member of the aforementioned Legion of the Dead she owes a Heroic Sacrifice and that she'll be leaving for her Calling as soon as the current crisis is over. If she survives to the ending her epilogue either has her follow through on that, or mentions that either the Warden becomes really good at finding new crises to persuade her to stick around just a little longer or she tries to leave but always ends up coming back to the Warden since she's one of his/her True Companions.