Of course winning a domestic league title is one thing and winning the Champions League title is another. The Champions League is the biggest challenge for even the best teams in Europe. Guardiola made a big effort to achieve that title with his usual game plan and strategies for years.
Maybe if he made slight changes according to their opponents over the years they might have won this title earlier who knows. But finally his system worked out and they won the title. It wasn't easy though as Inter resisted them really well. Guardiola will be remembered as the most successful manager in team history when he leaves.
Guardiola has the potential to repeat this achievement several more times, and he will most likely strive for this, but even without this, he will go down in history as one of the best coaches of all time. Guardiola has created a great team, I don't know who will be able to resist them in the Premier League. There are a lot of good teams in the Champions League who will try to counter him, but if Guardiola could repeat the success of last season again, that would be great.