According to the latest discoveries, Apple could provide the framework VoiceProvider to developers of custom speech synthesizers. This could also be debuted on iOS 14.
Speech synthesizer extensions would be offered with apps download from the App Store. They could be used by the system to replace Siri's default voice. A valid alternative for languages and dialects that Apple still does not support. This could potentially extend to an API for the HomePod in the future. But that at the moment there is not much information on this subject.
This frameworkVoiceProvider seems to be still in the early stages of development. It is therefore possible that it is not ready for the launch of iOS 14.
The first beta of iOS 14 is not expected until June during WWDC 2020. We will then see if there are more traces of this framework at this time.