Thank you very much for all your feedback provided so far. One of Fisheye/Crucible developers created a plugin providing this functionality, you can find it on Marketplace:

https://marketplace.atlassian.com/apps/1219472/mandatory-reviewers-for-crucible

At the moment we do not plan to bundle this plugin into Crucible, we would love to hear your feedback first. Some of the things we would like to discuss are:

• mandatory reviewers configurable per project or per review?
• if per project then would you prefer groups or permission schemes?
• if per review then how you'd like to distinguish them from non-mandatory ones?
• how do you see having mandatory reviewers vs review moderator?
• how many mandatory reviewers must complete the review (all / one / N)?

Feel free to try it out and comment under this feature request. The plugin is compatible with Crucible 4.3 and higher.