The perfect fifth member, the one that actually completes the cast, is Sally Acorn.

Here's why, for you idiots who didn't grow up with the good stuff.
Sonic: The rebellious 90s American teenager. He's got that Bill Clinton "break the rules, but get the job done" energy. He's freedom and attitude.
Tails: Kid Sonic. He's the little bro who wants to be as cool as Sonic. He's a gearhead, not some super-genius nerd. He can build a plane, not a supercomputer.
Knuckles: The Authority. He's the conservative stick-in-the-mud to Sonic's freewheeling liberal. The neo-con who guards his rock and distrusts outsiders. He's the perfect foil.
Amy: Girl Sonic. A cool rebel chick in her own right, but she's too similar to Sonic.

So who do you add? You add the opposite force. You add Nega Sonic. That's Sally.

If Sonic is the rebellious teen who acts on impulse, Sally is the mature, knows-best leader who makes the plans. She's the strategist, the one holding the line. She's the only one who can actually put Sonic in his place. She's the mommy-dommy peak that a guy like Sonic actually needs, not a fangirl who just copies him. Opposites attract. It creates tension, it creates drama, it creates a real story. That's the kind of strong leadership the team needs, not the chaos we get from Sonic Team's terrible writing. It's the kind of leadership this country needs, frankly. Dark Brandon gets it, Sally gets it.

This was the peak dynamic in SatAM and the old Archie comics before they let hacks like Flynn ruin everything. They had the perfect formula, a truly American cast of characters created by people who understood what made Sonic cool. But no, Sonic Team had to sideline the real heroine for their own bland, uninspired creations. It's a damn shame.