maybe if they want you to change to zss
I think that'll be done via Samus's story, i.e. she'll be Zamus for a while and then at some point don her power suit/perform a one-shot final smash.
I suspect that one of the two following situations will occur:
either
A) In SSE, there are stages that are just straight up battles (akin to many of the battles in Adventure Mode of Melee) on regular stages - these could easily incorporate Final Smashes.
or
B) In SSE, there will be a crucial point (or points) in the story where Final Smashes are achievable in order to increase the dramatic tension of the story - as the stakes grow higher, the special moves grow grander and stronger! Maybe the Smash Ball will even factor into the story somehow.