Well, in between assignments and getting excited about all these teasers and the Origin one-shot... so here's my thoughts.

I feel that the Phoenix: Endsong mini should be the launching point of her rebirth. She returned to the WHR at the end and I think she should just come straight out of there... having most of her pieces returned of their own accord. However, she will have to physically come out of the WHR to collect those pieces that have been prevented from returning to her by themselves, for example - the pieces held in the Cuckoos diamond hearts, and the ones that Korvus (? is that the right character? I didn't follow the Shi'ar storyline) and Rachel still wield. I'm not sure of the status of Madeline Prior's powers at the moment but that's about all the pieces still floating around (I hope).

The whole cosmic clone was a terrible retconn that many writers (including Greg Pak) have tried to bury, so it wouldn't be a good idea to have another cosmic clone (or a skrull replacement idea). Ideally, returning from a coccoon would be in keeping with the Phoenix-hatching theme but it wasn't as if she was last "killed" on a spot where she should be returned as an egg. My reading of the end of Endsong was that she just vanished... ascended into the White Hot Room (although some might say that it was Cyclops' eyebeams that vaporised her body - but since Jean/Phoenix thrives on his eyebeam energy, that wouldn't make sense).

Maybe the Messiah Baby is Rachel, maybe not... the baby's parents aren't definite, maybe she is a piece of Jean that "dropped" out of her to be a conduit for her return? I would much prefer a fully-matured Jean to return. If the baby is some sort of extension of her, have adult-Jean absord baby-Jean and thank Cable for looking after her.

Also, I definitely feel that Jean would not fit into the X-men titles long-term. Of course she would pop in once in a while to lead them into larger X-family stories but she's more in league with the Silver Surfer, Dr Strange, and other heavy hitters. But that may mean a lower profile for her in the long run, but keep her in the land of the living! Hopefully maybe her return could also help resolve Wanda's placement in writer's limbo too... not saying that Jean should reverse everything that Wanda had done, but maybe help in repairing the negative effects of Wanda's "madness", whether in healing the world's "no more mutants" issue or in healing Wanda's mind. I would prefer Jean to have to go away sometimes for "Phoenix work" and then return, but not by her dying (again).

If I knew how to keep Jean in the spotlight without depowering her or having ever-more-powerful villains pop up, I would gladly start writing her return!