would i like to see it? yes.
would it ever happen? no.
honestly, i just didnt think there was enough chemistry between the two.
It might not take anything in the way of chemistry. Illyria could simply be curious about her shell's "baser emotions and desires," and request a demonstration from her favorite pet. Spike being Spike...Well, let's just say it's been a long dry spell, and he's probably pretty thirsty by now.
Yeah - that would all practical and whatnot, but I don't think it would be particularly entertaining. It sounds about as exciting as a sex ed video. It's be easier just to queue up "The Miracle of Life" and be done with it. Besides, I think that at this point, Illyria would be completely disgusted by any and all of her shell's bodily functions. She already acts like the first flicker of emotion is disgusting.
Hmmm... now I have questions. Does she eat? Does she sleep? Does she, I don't know... go to the bathroom? Does she feel it when she gets cut? Does she.. ummm... I don't know... have any normal bodily functions at all? I mean, she hollowed out all of Fred's organs so that she could take over, right? Those systems don't work anymore. I imagine that that would include sensory organs, too.
If Illyria even can feel anything, I think she has years -- possibly decades or even centuries -- of humanizing to go before she'd ever let anyone touch her that way. For now, it would seem too human... and therefore disgusting.
Besides, Spike doesn't lust after her. No one but Connor does, and she picked up on that right away. The rest all see her as sort of a thing rather than as a woman.