She probably will play some part within DA: II however if on your Grey Warden, you made her fall in love with you, I doubt Hawke would be able to get with her, also if you selfishly chose to accept her offer and allow her to have your child bearing the Old God's essence within it, this could become quite a lot of trouble for you in the future because there is no telling what power the child posseses. It was tainted after all, and there is also Flemeth, even if you killed her in DA:O, Morrigen even mentions that she may not even be dead, it would just slow her down is all.
DA: II has already confirmed that it was span across 10 years and since we play a character called Hawke, who survives the Blight when Lothering was raided clearly tell us it's past the timeline when Lothering was destroyed and also bearing in mind that stopping the fifth Blight obviously did not take the Warden 10 years to stop, means that if Morrigen had your child he/she will be around 8 or 9, possibly even 10 if we do see them in DA: II.
Bearing that in mind there is always the possibility of Flemeth taking over Morrigen, evidence supporting this is looking at the concept for her for DA: II, she looks younger, one explanation could be Flemeth came back really pissed and got to Morrigen in the end. Providing your listened to Morrigen's request and slayed Flemeth. If not then it made it much easier for Flemeth to get her.
But bear in mind this is just one specualtion, I certainly don't want Morrigen taken over by Flemeth, but you all have you admit that would explain Flemeth looking younger.