In Skyrim, we are told that Hagravens are witches that have begun a transformation, giving up their humanity for greater magical power. Do you think that Harpy would be the next phase, as in Daggerfall it was implied that a similar transformation of humans created them? And is there a phase beyond Harpy that is fully bird and even more magicaler (Magicalest)?

Harpies look like hagravens gone wrong to me. Hagravens tend to be powerful witches, sometimes with a band of human followers. Harpies were regarded as vermin and they didn't cast spells in Daggerfall.

I don't remember that part. What book was this in, or was it dialogue?

I think it was a quest were someone says that her mother is going to change in to hagravens and you are ask to stop that.
About idea itself i like it, but i find it unlikely to be true, for one thing harpies seem in eso at least seem much younger, while hagravens are all, well hags.

An interesting query. Looking at the UESP entry on the holiday Riglametha (http://www.uesp.net/wiki/Lore:Riglametha#Riglametha), a holiday celebrated in Lainlyn, Hammerfell, it seems that "Daedra worshippers" in the area were turned into harpies for their blasphemous practices. Lainlyn seems to be close to High Rock, so perhaps one of High Rock's many Hircine-venerating witch covens were active in the area and maybe local folk either disrupted a mass hagraven ascendance ritual with or without Divine help or the witches simply did something wrong.

This is all conjecture. Though, considering the mention of Daedra worship and Hircine's confirmed involvement with hagraven gaggles and the creation of man-beasts, it seems quite plausible that hagravens and harpies have similar origins. Regardless, if the harpies were once mortals, it seems they have become their own species since then.

Maybe we'll get more information from ESO quests with the horrible creatures involved.

http://www.imperial-library.info/content/ghraewaj-and-harpies