Currently, Manchester City and Real Madrid are the two teams that are the favorites for people to win the UEFA Champions League next season if you are talking about teams that are qualified in terms of performances and squad in the team. Arsenal should focus on taking the Premier League from Manchester City because it is what fighting for. It will be a shame that at the end of all season, they still can't win the Premier League every time they get close. The only reason Manchester City will not put much interest in the EPL next season compared to how they will be looking at the UCL title is that they need more of the UCL cup and feel they can easily win the EPL.
I don't think it would be prudent for Arsenal to focus on only the premier league and not on the champions league. Arsenal is a big team and a big team fights for all the trophies available in the season. Focusing on one from the beginning of the season might be catastrophic because if they lose out on that one then they're left with nothing. Great importance should be placed on the major competitions which are the league and champions league. They can use the reserve team or second 11 to play competitions like the FA cup or caraboa cup if they're not facing a top opponents, but the champions league and League, should be their priority.