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.