Chelsea have seen steady improvement since Maresca's arrival, and their success in winning the Club World Cup should be a good indicator that they can do the same in next season's Champions League.
You're right, as you said, Chelsea shouldn't be underestimated. Personally, I'm quite confident in Chelsea, and they deserve to be placed among the favorites to win next season's Champions League. If that happens, it wouldn't be a surprise, as Chelsea have a pretty strong squad right now.
Since Maresca took the head position at Chelsea, it has been a progressive growth, one that has been consistently reviving the team in almost every area where in they were dieing off, I love it that now they have a trophy that has added to boosting their confidence enough for them to even do better in this new season at the premier and champions league but I do not trust them enough to win the champions league but may possibly attempt the premier league and finish decently.