Is there anything after the Boss level?

There are not yet any new jobs or job tiers to unlock after you've completed the Boss level. Zynga has not yet revealed any specifics about what future updates might entail.

In the meantime, you can work on:

• Getting Level 3 Mastery for all your job tiers
  • Ideally you should have been doing this all along, but if not this is certainly something to do.
• Completing all your collections
• Completing all your achievements
• Racking up successful fights, robberies & hitlist takedowns
• Keep leveling up
• Equipping your mafia with the best loot available
• Join a clan and participate in clan activities
• Help other players

It really comes down to your own play style and what you want to accomplish. Do whatever you enjoy! There's plenty of action to be had now that you've got so much experience under your belt!