Morrigan and Kieran's future

But .. I've made the poor guy Tranquil   because I didn't want to let a Dreamer loose on Tevinter 

Nah, you can if you do it right. But I don't think BW intended for things to carry forward with the series like they did. They've learned their lessons and handled things pretty well with Inquisition.

Yeah he has three different endings I think but if BW wanted to bring him back they could just handwave all of them into apprentice in Tevinter by claiming DREAMER MAGIC. His return is unlikely but imo it's a stronger possibility than seeing Kieran again.  

It came down to nothing.
When they made the option to have the OGB in DAO it's obvious they didn't plan far ahead what this was going to mean to the story in the future.
If Morrigan had the OGB, Kieran is free of that soul now. Whatever that even was because the game didn't really go into details what was going on with Kieran. Did he have two souls inside his body? And if so, wtf.
So yeah, either way it was removed and he is free. OGB story is over and didn't mean anything.

This story wouldn't make it past Ostagar if it was tabletop, considering how Morrigan introduces herself many a character would be justified in taking her prisoner rather than following her blindly to her 'mother,as for Kieren he doesn't play much of a role at all in the game. Its probably unwise to expect much movement on this, given that his existence is optional and dependant on your choice in Origins. I guess we'll just have to wait and see, but I'm not holding my breath for answers on this - Bioware rarely commit much resources to characters once they become optional.IMO, that scene is probably supposed to be the resolution for that character. I doubt either he or Morrigan will show up again for a while, if ever.
Imho the best choice would be a new start with new ideas,characters and with more creativity for this Franchise,seeing how they dragged for too long this Mythal/Morrigan subplot.

You know Flemeth says that but the clothes she was going to give her on return did have the ability to lessen her will power so.....

I was never interested in this subplot since this child has never existed for me and so he has no future and i'm glad BioWare didn't gave to him any critical role since that would have been pretty much detrimental for me and all the players who negated the Dark ritual as well as the new players who never played DAO.

As for Morrigan i already said my opinion on her long ago in the official BioWare forum,I despise her like I never did with anyone in my whole existence and the DAI main plot should have never and ever forced her in my way and in mine Inquisition since there was no need of her for anything in the whole game.

Now that Gaider is finally gone i hope that if DA4 will ever be released  she will not be in it with her lousy plot armor.

I didn't forget about Morrigan and with all the resources at my disposal i can make her life in Thedas a living hell with the best assassins of the crows of Antiva after her day and night
(I even placed Briala as empress and dismissed  lady Celine in order to be sure she has no allies) so she better fear my warden and think about how to survive against all her enemies rather than being in DA4 to bother my new PC.

I remember this character Morrigan,she was first seen in DAO.
Long time no see that game(7 years?!) but i do remember that there she was a sociopath and a little bit insane especially for her gaining pleasure and approval from the suffering of others or even outright advise to cause pain to others.
She was even scheming to use the protagonist to save the same beast who caused the blight and killed thousands of people via darkspawns,i wonder why there was no option to kill her in the castle when she returned without permission in a private building solely to use the hero for her weirdo plan. Guess that they wanted to reuse het for another game and decided to adopt a plot armor.

I don't know who this Myhal and Kieran are however,crazy mages like her i guess?

Mythal was one of the leader of the elven pantheon, who in short was murdered by fellow Evanuris for reasons. Later on in history, Flemeth accepted her wisp form into her body. Vengeance among other things seems to drive them. Kieran is the potential son of Morrigan and the Hero of Fereldan/Alistair/Loghain. It is possible for him to not have Urthemiel's soul if the HoF had sexual intercourse with her, break off their relationship and did not perform the Dark Ritual. Flemeth ended up taking the Old God's soul anyway.