The Champions League: Manchester City’s final frontier. After a clean sweep of domestic trophies in a staggering 2018-19 campaign, and after claiming five of the last six competitions available in England, only Europe needs to be conquered by Pep Guardiola and his juggernaut team. The manager knows it. At a Lionel Messi-led Barcelona, Guardiola guided his boyhood club to European Cup wins in 2009 and 2011. The Catalan has managed seven times in the continent’s blue riband club tournament since and failed to win it again: once with Barça, three with Bayern Munich and three with City. The equation is simple for Guardiola to identify and weigh up. After claiming the last two League Cups and this season’s FA Cup with Saturday’s 6-0 trouncing of Watford, next season is surely the moment to forget pursuing these lesser trophies and instead marshal the extra physical and mental energy required for a tilt at a triumphant Champions League campaign. The Watford victory was game number 61 for City in a season that started in August. They fell only three short of the maximum number of fixtures possible. “I know we will be judged at the end on whether we win the Champions…