One alternative could be to focus not on size but on the structure of the institutions and discourage complex structures. The problem I find with the metric used for measuring the systemiticity of financial institutions is that it is not an absolute metric but a relative metric. It calculates the systemiticity of a bank in relation to the global sum of various metrics.
In this relative approach, there is always a possibility that a bank, despite growing rapidly, and becoming riskier, might escape the SIFI regulation, if the entire banking system also becomes riskier due to which its “score” remains unchanged. I must add that the methodology devised by the Basel Committee is the best available at the moment.