If MS Flow is successor of the SharePoint Designer workflows, then the basic feature available in designer workflows to enable the service account to create and publish the workflows and all users having access to the list/library to trigger/start the workflow should also be available in the FLOW
Basically the idea is to have single service account to create and publish the flows and all end-users should be able to trigger/start the flow provisioned by the service account.
These users should not be the owners/contributors of the Flow, however if a FLOW is attached to list/library they should be only able to start the flow.
For E.g.: All users should be able to manually start the “Publish Document” Flow attached to the document library
Hope MS take note of this and allow all users [with appropriate permissions] to trigger/start a flow provisioned by the Service Account.