Again, it might have worked better if they'd found a way to incorporate the Eve idea into the fourth/fifth season, as a sort of: "Oh dear, the wheels have really come off and shit is about to go down" thing rather than trying to make it stand on it's own. They could have then substituted Lilith for Eve. It would've certainly worked a lot better, given how Lilith is, canonically, the mother of demons/beasts, if I remember correctly. At least in my humble opinion.

They didn't seem especially fond of female characters on the show, no. They either pop up for an episode or so only to disappear into the aether afterwards, or they're killed off brutally. Which is unfortunate. I was rather fond of a couple of them.

I certainly wouldn't mind seeing more of them. Movies like X-Men and Transformers are fun and all but sometimes it's nice to be caught off guard. If only to stir things up a bit.