I would like to suggest there is a restricted view and idea type option. We want to use one idea project for everyone but some teams need ideas private for sometime or forever.
How about adding a dedicate field "Private" (checkbox or multivalue - if multiple restriction needs are needed) and filter those ideas out of the views you are sharing with others, leaving "restricted" views in some other nested group?
It's kind of workaround but could help limiting the amount of info directly accessible to some stakeholders...