Most NBA teams' starting fives are three or four deep. They have the majority of the pieces in place to make them a decent or even good team.
What most teams need is that one piece (or two, in some cases) to plug into the starting five to get over that hump that seperates decent teams from good ones and good teams from great ones.
These pieces do not necessarily need to be All-Stars or superstars. Sometimes, a good role player will complement the other starters to make the team's starting five ideal.
The rules for making this list—I can add at most two players to a current NBA's team's starting five. The player(s) I add through free agency must fit under the cap. If I add a player(s) through a trade, it must work in ESPN's Trade Machine.
Free Agents can show up on multiple rosters, traded players also. For example Nene can be ideal for the New Jersey Nets and the Denver Nuggets.
Feel free to make recommendations of your own.
Here are my ideal starting five: