Audi A6 C7 quattro - what are some must-have upgrades and modifications?

Hello fellow Audi enthusiasts, I'm looking to upgrade my trusty C7 A6 quattro and wanted to get some advice on must-have modifications. Specifically, I'm interested in learning about upgrades that can improve its performance, handling, and overall driving experience, while also being mindful of any potential costs or compatibility issues with the vehicle's original components.