A350 MRTT would be a perfect future tanker transport - particuarly as Boeing has already shot itself in the foot by stating that their 'plastic plane' 7E7 doesn't have the 'configuration' to be a tanker transport KC-7E7. Bit of an own goal that - no doubt done to promote the somewhat dubious KC-767 programme which is on the buffet right now....
And don't forget that common type ratings for everything from A318 to A380 (with differences courses) would probably also include A350!