Call it Juventus, Milan, Napoli and Roma. It is certain that these four teams will be very tough competitors for Inter next season. Because of these four teams, you could say that the teams in Serie A are the most active in the transfer market this season. So it is clear that these four teams are very serious about preparing their teams for next season. Therefore, Inter really has to be aware of this.
To be honest, I haven't received any official news about the players they will recruit next season. Juventus and AC Milan are indeed tough competitors for Inter Milan for next season. I can say that this season Juventus and AC Milan cannot put pressure on Inter Milan who dominate Serie A. But if they are active in the transfer market this season I think they will really be a tough competitor for Inter Milan.
I don't know how AS Roma and Napoli will fare next season. As Roma will be better next season because I see De Rossi is a great coach. Napoli performed very badly this season. I'm not sure they will be contenders for the Serie A title next season.