There are currently settings regarding the Subscriptions as to whether the users automatically added as participants based on their subscriptions can have their roles changed, be removed from the reviews, etc.
It would be nice if this setting related to subscribed participants could be set on a per-subscription basis. It would also be good to have an "Mandatory Enforced Role increase only" option to allow those subscribed as Observers to be promoted to Reviewer or Moderator, and Reviewer subscriptions to be promoted to Moderator.
We currently have some new developers that are required to have specific mentors as reviewers on their reviews. In other cases we require certain users be moderators on changes to specific files. Most of our subscriptions have users set as observers though, and they'll occasionally be added to reviews they aren't needed on (test reviews, out-of-group reviews with conflicting file names, etc). Additionally we'll very frequently have cases where subscribed users that are automatically added as Observers are chosen to be promoted to Reviewer or Moderator on some reviews. With this mix, it would be useful to be able to specify the few subscription rules that are mandatory, allowing only role promotion, and leave the rest as optional at the discretion of the review creator.