In the spec custom elements can inherit from any html element you wish. Which should allow you to only have to add the bits you need to add and let the rest be handled by the native implementation.

The one exception is Safari which has been slowly getting more and more special over time when it comes to web standards but is still relevent. If Safari found their way to supporting it then you shouldn't need to completely re-implement a button or combobox and instead just improve the native versions.