If next season Napoli brings in several good players in several sectors, especially recruiting quality midfielders, then Napoli will again become a serious challenger in the race for the Serie A title next season. With a good midfielder in the Napoli squad, Osimhen performance will improve again and he can score lots of goals for Napoli.
That's the only way Napoli performance will improve because they have really go through a lot of struggle on this season, imagine Napoli could not even qualify for the fifth position of the league table and even the smaller clubs like Torino, Fiorentina and Lazio was even more better than them on the season, so they really need to upgrade themselves because it will be very bad for them if they are not able to qualify for the champions League on this season they are about to enter, so like you said if they are able to get more good players especially midfielders and few strikers they will actually improve but however in terms of Osimhen I don't know if he will still remain on the club because rumor has it that he could be leaving anytime soon but I don't know how true is that.