Diameter is designed to be extensible. 3GPP has used this extensibility to defined may 'interfaces' (Diameter applications) which are widely deployed now. (e.g., Gx - TS 29.212)
There are proprietary extensions too. But idea is to use existing commands and AVPs to the maximum possible extent, add new 'optional' AVPs as needed. But if existing commands can't serve the purpose, you will end up defining new application.
There are tools that make this easy - and we have one :) Can explain more if you have specific questions.