The melodrama of S.I. secession did not play out all the way. Former Governor Cuomo refused to outright squelch the idea despite hard city opposition.
It was Giuliani's election that really killed S.I. secession for the time being. If the next Mayor of NYC reverses Giuliani's policies and resumes the former isolation of S.I., expect to see secession advocacy re-emerge.
As to the question of whether other parts of the city might want to secede if S.I. does, remember that parts of eastern Queens (including, significantly, predominently minority parts of the borough) also were interested in secession.
It would completely change the NYC-NYS political equation.