I'm going to second these suggestions-especially Carlos Ramirez. A Carlos would be AWESOME. And Angel. I'd really really love to see Angel.
Want/Need:
Sanya from Dresden Files. There is so much potential for plot involving the Knights of the Holy Cross, particularly if Lauren goes with the plot I suggested for the sword that is currently without owner.
Jared Kincaid from Dresden Files-because, holy wow is there a lot of plot for him in the wake of the revelations in Changes about his part in Harry's shooting. Though Murphy would be a force to be reckoned with if we ever do get a Kincaid.
Need:
More angels! Or any angels really.
A slayer. Buffy or Faith. Obviously, as mentioned above, there's a lot of plot for a Faith if we get one. And I know we technically have a Buffy, but it would be nice to see Buffy react to Giles getting hurt and all that.
Charles Gunn from Angel-Um, yeah, because he rocks. And he's a hottie. Also, depending on when he's taken from, his ties to Wolfram and Hart could majorly tie in with plot.