Teams/S4B Online/S4B on-premise : What will be the migration path ?

I think more details will be forthcoming on these different scenarios, migrations, recommendations etc. 

Right now, Microsoft is encouraging all Office 365 customers to start using Teams, independently or in parallel with Skype for Business.  Microsoft also says for those customers in the process or are planning to deploy Skype for Business:

"Customers should evaluate the Teams roadmap to determine if capabilities will meet business needs and timing requirements. If so, they should adjust strategy to deploy Teams. Planning and infrastructure investments such as network assessments will accrue to Teams deployment. If not, customers should move forward with Skype for Business."

Here is the specific guidance that's available right now including a https://docs.microsoft.com/en-gb/MicrosoftTeams/faq-journey, in case you haven't seen it:

• https://docs.microsoft.com/en-gb/MicrosoftTeams/prepare-teams
• https://docs.microsoft.com/en-gb/MicrosoftTeams/pilot-essentials
• https://docs.microsoft.com/en-gb/MicrosoftTeams/guidance-skypeforbusiness
• https://docs.microsoft.com/en-gb/MicrosoftTeams/continue-journey

Stuff like hybrid I haven't seen mentioned yet.  Microsoft Teams won't have all the Skype for Business audio and PBX features until the second half of next year anyway I understand and these timescales may change. Depending on what stage you are with the migration and stuff like priorities, user impact, costs, it might be worth continuing the migration and then at a later date, move to Teams, when everything is in place. 

Hope that makes sense and is of some help.