Why not 737(7)-738-739 and 767? Common short/medium haul backbone to cover everything from lower end like NCL-KRK, LBA-DUS etc. with the -700's, top end UK-Balearics, Canaries, Spain with the -900's and everything else inbetween and alongside with the -800's.
The 767's could cover everything from summer PMI/ALC/TFS shuttles, EWR charters and long haul jet2holidays