@photex It's so bizarre to me that no one's come along and tried to recapture it. It's not like what made XSI so special is undefined and unknown, the blueprint is right there! And hell, your post as small evidence, there's still a built in user-base pining for it, even now, so long after it was taken out back by Autodesk.
Anecdotally, I've talked to *so* many ex-XSI users that still want the chance to bring it back somehow.