The hardest part to trying to add substantial talent in a trade isn't having the assets, it's having the other team willing to actually part with an All-Star or high level player. That's why the Kevin Garnett trade years ago was so huge. Players of that level are rarely made available. And when they do they come with some warts.
Fans are down on recent All-NBA All-Stars Ben Simmons and Pascal Siakam based on recent poor play and injuries. But they are examples of the rare top level players that are either currently available or might be soon.