Sooo.... Grimmjow!Xander? He's got the grin for it.
Other than that... Ulqiuorra!Xander? X has been known to dabble in murderous logic, on occasion. (Amy's Mom, Angelus, et cetera.)
Only two Espada worth the effort, in my opinion. ^_^
He'd probably end up with some level of spiritual power, some basic abilities like Cero, and a sealed Zanpakuto. Maybe he'd feed on Vamp essence - they don't have souls, per se, but they are Demon-powered bodies. Given that Hollows eat each other for power, I could see the same happening here, if Xander was desperate/hungry enough. That would freak the fuck out of the Scoobs, though, unless he disguises it:
"Eat them? I didn't eat them, silly! I purified them with my Soul Cutter." :evil:
The real fun would start when he dies.