Open play test servers.
There’s 2 ways to go about things here. You can test and keep it internal, doing your best to find bugs/exploits and squash them before release. This obviously means you can surprise the community like you did with the class skill tree patch - but it also means you’ll miss obvious exploits, like the infinite empowered psionics + smite bug.
Or
You can open play test servers for upcoming content that can be used to generate hype, create a sense of community involvement, provide crowd sourced play testing and quality assurance, provide meat to the playerbase between patches, will keep expectations tempered as it is explicitly listed as a play test, and avoid implementing absolutely game breaking exploits by mistake like the smite exploit! Hell you can even throw new cosmetics on the play test servers and get feedback before you officially release them and run into issues like having to push hot fixes for them.
You guys would be looking at a fantastic win and a great way to do what you do best, polishing your game to perfection. Simply state that player progression will not transfer to the live server and start players at level 30.