You could rebuild Arsenal to what it used to be in 2003-2004.
With Napoli there isn't really much to do, they are already a top tier Italian team.
Try Torino FC, they used to be one of the best teams in the world around 50 years ago and then all the club members died in a plane crash. Perhaps if they didn't lose their asset the way they did, they'd be a top Serie A time now. All those top players dying without being sold or winning the club some money, the club could have gotten better and better overtime with all those wins but instead everything just disappeared.
