From what I've read all five of the 757-300's are being transferred to German sister airline Condor and in return will be swapped for six or seven A321's that will be joining Thomas Cook UK.
The Thomas Cook fleet seems to be very complicated, as there is a lot of swapping around of aircraft with the sister German, Scandanavian and Belgian fleets. With these flying odd flights to and from the UK. For example Thomas Cook Belgium operated the Palma-Luton-Palma flight this summer.
They also seem to wet lease a number of other aircraft in the summer months from Avion Express and SmartLynx, which are usually A320's and I assume this will continue next Summer...
You also have Air Tanker continuing to operate two A330's for Thomas Cook next year.
For example they are opening a new Luton base next year, and the routes are showing to be operated by an A320 on the seat map when booking a flight... So I assume this will be a leased bird, which makes sense when opening a new base, as it minimises the risk with not having to employ new Thomas Cook staff. as these will be supplied by the leasing company and will be flown by a smaller aircraft than the A321, in case the base doesn't work out or has lessor demand than that is required of an A321.
To add to the complications, they are have also entered a deal with Air Transat to share aircraft during each airlines low season. So Thomas Cook will send A321's to Canada in the winter, when Canadian's want to escape to Florida, Mexico and the Caribbean. I assume Thomas Cook will receive a few of Air Transat's metal in the summer month's, maybe an A330 or two, but I thought Air Transat's long haul fleet were busy flyng to and from Europe in the Summer?
Also the A330's of Air Transat are a rather cramped nine abreast and will be interesting to see if they get modified to match the UK fleet with eight abreast seating or remain as they are.
The problem Thomas Cook have with their long haul fleet is, they have an inconsistent hard product, as you have their own fleet with Premium Economy and offer Seat-Back TV's throughout. The Air Tanker fleet don't offer either and if the Air Transat A330's enter the fleet for the summers and are not modified, this makes the hard product even more difficult to match customers expectations.
Air Transat's short haul fleet is made up of 737-800's so unsure if we will see any of these flying for Thomas Cook in the Summer months, and these are due to be replaced with A321LR's in 2019, which will be more compatiable with the UK airlines own A321's.