Man City are probably the best club in Europe right now, but don't forget how they only managed to push Arsenal out of first place at the end of last season... So there are strong clubs in the EPL now too... I think that next season Man City will not be as easy as last season and I don't rule out the possibility that they won't be able to win again...
If the top clubs in the EPL still maintain their old players and don't add or overhaul their team squad it might be a bit difficult to compete with Manchester City, so far Manchester City already has several quality players even though they have several players who have left but they are adding players new additions to complement the departed players so the team will remain solid.
Being a consistent club in defending titles is not easy, although I know for Manchester City it is not that difficult at the moment. But still, the ups and downs of challenge after challenge will complement each club's journey. With the club's spirit overhauling its squad proving they are serious about being a worthy contender next season. If you focus on the Premier League, there is no need to doubt the dominance of Manchester City, but the Champions League will be difficult to predict.
You could see it either way. For some clubs confirming or repeating past results is tough because key players leave the team and they can't replace them equally often times or they performed on an unsustainable level for a year. But teams like Manchester City have this winning DNA. For them it is more likely to perform as good as last year as it is for them to have a weaker performance. But is it as likely for Napoli to perform the same way in Serie A? I would say no. Manchester City has most of the key players still there, many of them want to go down in history and they have Guardiola who understands how to fire them up again for another season.