I would really like to be able to use any Global Rank field type as the secondary sort on my Kanban boards. I can see this happening in one of two ways:

1. Allow JQL statements like "ORDER BY priority DESC, Rank ASC", but require the user to specify the name of the rank field (possibly a prepopulated dropdown of the existing sorting fields in the query/filter). The problem with this approach is handling the UX for drag'n'drop reordering, which would be sensitive to the context of the primary sort.
2. Create another Rank field type, or allow the current one, to default to a particular value (like 0 or the maximum integer value) instead of auto-incrementing the value each time an issue is created. This would allow the system to treat sorting statements the same - "ORDER BY Rank ASC, priority DESC" - while allowing a "default" ranking to take place until the rank value is set (by dragging and dropping a card higher on the list)