The problem with Superfast X is she has roughly the same freight capacity as Nordica (in fact Ferries 2014 lists Nordica as having higher capacity than Stena's Belfast Superfasts). It's easy to forget that despite her small size Nordica does swallow quite a bit of freight. There isn't much out there with larger capacity than 2000lm and significant passenger accommodation, never mind out there and available. The only way Stena could boost capacity in the short term from what I could see is either bring in a 3rd ship (as they have at Birkenhead) and somehow work her into the timetable, or deploy a Visentini themselves. The scarcity of tonnage at present means that prices are going to be at a premium. We could also see tonnage displaced from the Baltic with MARPOL making some operations unviable. Much of the tonnage there is of 80's vintage and probably not worth upgrading. With instability in the Middle East continuing it's not inconceivable oil prices will continue to rise as well.
Regarding tonnage under construction nothing springs to mind. The European yards which spring to mind (STX France and FSG, Meyer, Fincantieri, etc) appear to have pretty full order books as well, thanks in no small part to the cruise industry. STX Rauma was probably the most likely European yard but it has closed down. The Rodin/Berlioz/Star design could have been a good staring point for a new Dublin ship IMHO, with less lead time being an off the shelf design. I wonder how easy it would be to add significant passenger accommodation to both Epsilon and Stena Nordica btw. With nothing official yet on Superfast X it's still possible given the current scarcity of tonnage Stena RoRo could get an offer they can't refuse - Blue Puttees and Highlanders are an example of this, originally I believe earmarked for Belfast.
As for Contentin, I think it is likely that once her charter is up she will be swapped for Stena owned Etretat. It would make sense for both companies - Baltica is an odd ship in the Stena fleet whereas she has a lot in common with her BF fleet mates (particularly Amorique) whereas Etretat is an odd fit at BF while the majority of her half sisters are owned by Stena. The charters also expire around the same time from memory.
The problem with Barfleur is she probably won't be available any time soon, Bretagne looks to be the ship most likely to be displaced if everything goes to plan with PEGASIS. I am not familiar with IF's figures for Pembroke, but on the Stena side it would appear they have excess freight capacity - though this is an area they are actively trying to improve following the adding of Celtic link to their empire. Perhaps a smaller vessel could work for Pembroke, I would have said Calais Seaways where she to become available might have been a decent option for Pembroke following an extensive refit. However I think she will end up back at Portsmouth if DFDS continue with the operation there.