Update 29.03.2021: Following the publication of this story, Stena Line has released a statement confirming that a final decision on the deployment of these vessels has not been finalised. It is reproduced in full here.
It appears that Stena Line’s next new ferries will be deployed on the Nynäshamn – Ventspils route. That is according to a now edited article by Stena AB CEO and majority shareholder Dan Sten Olsson. The two ships, which are currently under construction at the China Merchants Jinling shipyard in Weihai, are a lengthened version of the standard Stena E-Flexer platform as used in STENA ESTRID, STENA EDDA, and STENA EMBLA.
Writing in the Stena Sphere’s internal magazine ‘Sfarnytt’, Mr Olsson also states that Stena is “contemplating an investment case a for battery pack” to be fitted to the vessels. This would allow the ships to run on battery power part of the time while at sea in order to reduce exhaust gas emissions. The design already incorporates a shore power connection. This will mean the ships can switch off their engines whilst alongside at a suitable port.

The as-yet unnamed ships are both expected to be delivered during 2022. Their arrival will presumably displace the slightly smaller STENA SCANDICA and STENA BALTICA (the rebuilt former STENA LAGAN and STENA MERSEY) from the Ventspils – Nynäshamn route, should plans remain unchanged. It is likely ‘Scandica’ and ‘Baltica’ will then be transferred to another Stena Line route.
One possible candidate would be the Karlskrona – Gdynia route, which many had expected to receive the lengthened E-Flexers.