Unlike most legacy panels, Enhance is compiled code, it doesn't run in an interpreter such as PHP or Perl.
This is great for security but it does mean that it's not possible to support plugins in the same way as software such as WordPress and WHMCS where those plugins execute alongside the core code and have full access to the database.
The best way for 3rd party developers to integrate their product with Enhance is using the API. Existing integrations such as cpguard do this. However it means the 3rd party products need to provide their UI, either using their own web server or through a website running on Enhance.
It is possible in the future that there might be a framework for adding custom Javascript to the Enhance UI so that custom UI can be presented within the Enhance panel itself. This isn't immune to security issues because malicious Javascript could still capture user input.
It may also be possible in future to upload a script which is run on website creation, as the website user. This could be bash, PHP, Perl, etc. This could be used to integrate 3rd party software installers, to install custom frameworks on website creation, etc.