Would Superfast X not be suitable for Frederickshaven Goteburg or Frederickshaven - Oslo or a new route Belfast - Troon.Apart from the fact Belfast to Troon makes little sense due to it being just as quick (not to mention cheaper) to sail to Cairnryan and drive than to cross by conventional ferry, there’s absolutely no hope of getting a vessel the size of X berthed at Troon! European Causeway is about as big as will go.
Gothenburg to Frederikshavn is configured with one more passenger orientated ferry (the still extremely popular and well maintained STENA DANICA) and one Ro-Pax (STENA JUTLANDICA). Both are also purpose built for the route of course. Freight demand has increased and so a third vessel has been added (currently STENA VINGA). Shoehorning X in as a replacement for either of the main vessels just doesn’t make any sense - Jutlandica has about the same freight capacity and more passenger capacity than X. Danica has more passenger capacity. X is no freighter either.
The Oslo route is more of a cruise ferry route and geared towards that traffic. X is many things but a cruise ferry isn’t one of them. Saga also has a heck of a lot more passenger capacity than X. It’s hard to see where Stena could move X within the current Stena Line network to be honest.