Will a new workflow engine be used with Nintex for SharePoint 2016, or future releases of Nintex for SharePoint?
Currently there is no plan to stop using the Legacy Workflow Engine with Nintex for SharePoint.
Nintex uses the Legacy Workflow Infrastructure within SharePoint 2016, and there is no plan to move the on-premise workflow product over to the Workflow 2013 engine. There are several reasons for this product decision outlined below:
Extensibility: While the 2013 Workflow Engine is more efficient than the Legacy Workflow Engine, it is also far less extensible, and moving the on-prem product to the 2013 Workflow Engine would result in loss of functionality that customers have come to expect from the Nintex workflow product. This has been one of the largest pain points of migration to the Office 365 platform (which does use the 2013 engine), and as such Nintex has decided to use the Legacy Workflow engine as long as it is supported within SharePoint.
Additional Customer Setup: If Nintex moved to the 2013 engine, this would need to require customers to either have the 2013 Workflow Engine (Workflow Manager) configured prior to installation, or make it part of the product setup process.
External Access: In order to bring parody in functionality to the 2013 Workflow Engine of those features offered by the Legacy Workflow Engine, users would need to enable Nintex Live, and in doing so open the SharePoint environments to the internet for access to Nintex Live.
While there is not currently plans to move away from the Legacy Workflow Engine, should Microsoft deprecate the Legacy Workflow Engine, Nintex would need to re-evaluate moving to the 2013 Workflow Engine (Workflow Manager), or potentially implementing a different workflow engine in the future.