Don't get me wrong. I would have preferred your original suggestion with Ronnie’s baby that lived but her mental health deteriorating and the mental health route to be the focus for her character. That would have been a bitter sweet dramatic storyline, that wouldn’t have felt as over the top and ridiculous tragedy, knowing that she had the baby in the end. But under the circumstances, when everything was decided and that her baby would die and there wasn't going to be any mental health focus, her exit was the best one could have hoped for.
I am convinced this storyline was all about Kat when it was planned (Ronnie being impregnated by a comatose man being the biggest proof of that) and I suspect they changed it after the viewers’ reaction and turned it into a storyline that focused a bit on Ronnie too. Possibly by also bringing Sam into the discussions and asking her about the character. However by that point it was too late to change it drastically and do what they should have done in the first place, so they went with a compromise with a slightly fitting ending.
For me they can’t do anything interesting with Kat because she went full circle. She did have the troubled past and the longed for wish to be the mother of her daughter which were both requited and dealt with. Dragging that up would feel old. She was the unlucky in love, tart with a heart and that too was resolved and given a closure with Alfie.
Whatever they do with Kat doesn’t feel enticing. I can’t think of anything exciting tbh but maybe because I am not invested in her character at all. Maybe other people have some ideas but I don’t want to see a return of the Slaters to cushion Kat. It is too much, bringing back all these characters and the cast is huge as it is. It will either be repetitive and feel old, or they will go to extreme lengths like the baby swap to try and top what they did in the past with her. If they split her up with Alfie we will go back to Kat trying to find Mr Right and suffer etc, just like she did before she got her happy ending with Alfie. Keeping her with Alfie and also with a baby is limiting in terms of storylines for them.
She doesn’t fit in the show anymore. I think some things are best left in the past.
I may feel the same about Ronnie at some point, but in comparison Ronnie only has one side of her story told and never got her happy ending so her story can continue. I do however feel that if Ronnie was to return she should return at the end of her prison sentence, sooner rather than later. And she should not return as a tragic heroine. Otherwise she won’t have a place in the show at all. The show moves on, the characters she has links with will move on too and she might not fit in with them again.