Allow collaborators to upvote cards
A key element of story mapping is collaborative prioritization/prevalence. IE, understanding how popular a given card might be amongst the participants.
In an in-person setting, this is achieved either by asking participants to spend several minutes writing ideas on sticky notes, then consolidating similar ideas, or asking participants to "dot-vote" on ideas that resonate.
If Stories on Board allowed for live up-voting of cards as they are added, this would come close to replicating the in-person story mapping experience. Collaborators should have a simple mechanism to upvote a card (and ideally could add clarifying comments that would be visible on the card itself).