Having a look at the Saturday schedule for July I could imagine perhaps something like the 8am Palma and/or Malaga being subbed to A350, especially if the US market opens up as they'll have heavy local and connecting demand at that time. A quick Spanish rotation should fit between an IAD or a PHL, Aer Lingus do the same

As others have said there is actually little to be gained seats wise vs an A321 and you need more crew/fuel, so it'd need to be high yield fares on very full flights so maybe tactical subs on v full flights rather than a series scheduled in advance