Miscellaneous ideas topic.
It would mean he can't exist, to put it bluntly. For one, Alexstrasza already has a mate, and she's not in the habit of betraying. And since her mate is a dragon himself... Negi just can't be a half-dragon. At least, not to HER. She's much too faithful for that.
Secondly, Aspects don't have a real body to speak of. What you see is what they choose to manifest themselves as. And Nozdormu once implied that they can't really get kniocked up - it's Alexstrasza's power as Aspect of Life that allows herself to give 'birth' to dragons.
Third and **most important**, all dragons, even Aspects, find the human form quite disgusting, and will only use it when necessary. The mere idea of them having sex in that form, let alone with a HUMAN, is so impossible it isn't funny, barring total and utter character rape.
Well... Deathwing might do it, simply because he's crazy. But not Alexstrasza. And seeing how Deathwing is quite clearly male, that throws him right out, since we KNOW who Negi's dad is.
Sorry to rain on your parade, but as a major Warcraft fanboy, I am violently opposed to character rape towards its characters.