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The problem with having babies on tv shows is that they are babies, they do very little, are annoying, have to be considered before any action is taken and add nothing to the show (imo of course) so most tv shows age the children through mystical or time jump means, which can come across as cheap if not written well.
Would you want 20 year old Buffy to have a baby? Or would she be aged to 30 and have a 10 year old? How would it work in practice? It could have worked, for sure, but it would have changed the whole nature of the show and the Buffy, Dawn, Joyce relationship.
If that were the case then I wouldn't want her to be the Key. The show already has more than enough mystical pregnancies (and while I don't mind them going there once or twice, the two shows went there to about the maximum I could stand).
They could milk all sorts of drama from it, especially if say it was the child of Riley (gods forbid Parker) and Buffy, and maybe they argued about it before, Buffy worried about her Slayer responsibilities and how a child would be a target for fates worse than death...they do, with mandatory oh isn't the baby special episode, then baby (who is indeed targeted even sooner than Buffy thought) gets snatched to another dimension (and THIS is what causes the rift between Riley and Buffy that causes them to split rather than involving vampire whore houses, especially if they got into each other's way, and Riley desperate not blame himself blames Buffy who was still recovering from pregnancy, even implying she wanted this to happen which Buffy couldn't let stand, and this would also prevent her from chasing Riley and the helicopter he got on, even if she wanted to).
Then Dawn returns older (similar to Connor--who in this case should NOT have that as a plot arc) thanks to Willow and Tara's spell (using Buffy's blood to help track missing Dawn and know she's still alive, which establishes a mystical link between their blood that isn't sufficiently explained in the version we got), help explaining why she likes them more for rescuing her (naturally, the spell brings over other baddies as well, possibly even the essence of a Hell goddess that anchors in Ben, and now wants to link Earth to her home dimension in which Dawn, one with a foot in both worlds, is an essential key spell component). Given Dawn's upbringing (and especially if they're aware that some Hell goddess escaped to parts unknown) they could be understandably suspicious of her at first.
But they'd have to change so much...I think the writing on both Angel and Buffy could've gotten a lot better as a result (though really the writers probably needed a long vacation before continuing, skipping a year).
I actually like the idea of a Warrior Dawn, too, though I'd hope she'd change a bit more than Connor. They could throw in an ethical debate over altering her memories through magic to remove the trauma (something that could come back later). It should be kept in mind that plenty of kids raised in hellish war zones that would make many demons of the Buffyverse cower in terror that still go on in our world today can adjust to civilized societies without being psychopaths.
And with a little work they could even insert a humorous connection to how she was (at least by season 7) able to go to school because it reminded her of some demonic academy which made her appreciate what it's like in our world (and contrary to popular belief, which I say from experience, you don't have to go to school to have friends from school--in my experience, it was actually easier to make school friends since I was no longer part of the school status system once I dropped out and thus more were willing to be friends with me). It could also make visits by social workers a lot more stressful, though I presume Joyce (backed by Willow's reckless magic) could've helped a bit, though Dawn couldn't be seen as Buffy's daughter by the system as Buffy is too young to have a daughter that old.
I still favour the idea that Joyce was a Godparent to Dawn and when her parents died, Dawn went to live with Joyce and Buffy. Dealer's choice as to whether her parents died of natural causes or supernatural ones and at first, Dawn is just a normal teenager who is weepy because her parents died. But then we get some inklings that something is not normal. Insane people recognize her as the key. Little tears in the fabric of space-time show glimpses of other dimensions. Them we find these get longer and more powerful and beings from other dimensions begin to cross over. Buffy sees all this and brainstorms with Giles as to what is going on. Dawn sees some of this but does not understand what is going on. Gradually they get the picture that a demonic female hellgod is looking to use her to rip the fabric of space-time apart so she can return to her domain.
I quite like @Ethan Reigns idea as it's a way to introduce Dawn without having to pull a "she was there all along" pretence.
For Dawn as a baby, it would be a good way to introduce the concept of Nikki and Robin and also Buffy's worry that she would be killed such as Spike killing Nikki. Also it would be kind of poetic that Buffy names her daughter Dawn since sunlight is lethal to vampire just like Buffy is as the slayer.
Alternatively, they could've had Dawn and Connor be twins and the children of Darla and Angel. Similar to above, it's would be kind of poetic for Holtz to name the girl Dawn and for her to be the child of vampires. Dawn can escape the demon dimension to Sunnydale and Glory follows, only to realise that she prefers her own dimension but can't get back. Glory then realises she has to use Dawn's blood to reopen the portal, and generally follow the plotline of Season 5 minus Ben. Glory is successful and returns home but reality is being ripped apart as the result of the portal. Buffy tries to save Dawn, unfortunately whatever they were on breaks and the two go tumbling down. Dawn is sent back through the portal and it closes whilst Buffy gets caught up and dies like she did in Season 5. It's a mystical death so she can be resurrected. Dawn then reappears on Angel with Connor.