Right now in Bitbucket version 5.x we are able to set up Default and Mandatory reviewers for the project as a whole. Merge checks are also repository level. It would be a great addition if the User who creates the Pull Request is allowed to specify a custom set of Mandatory Reviewers for every PR that is created. 

There should also be a chance given to reviewed, where they can mark any bitbucket licensed user either mandatory or optional or with no reviewer. There shouldn't be a need to set the minimum level of review, this will also depend upon how many mandatory reviewers are tagged against each pull request.