If APD is scrapped in Wales, I cannot see any immediate reaction by airlines apart from a slow drift towards extra charter flights to medium - long haul destinations.
I agree, Thomson and/or Thomas Cook would probably be in quick with their standard long haul leisure combos but, excepting a wildcard like Norwegian, I don't see CWL getting routes like New York as there are unlikely to be enough first and business class passengers on offer to interest the majors.