Writing ensemble casts in single-player campaigns

You know how in most shooters and sandbox games, e.g. Call of Duty and Assassin's Creed, you only have one main-character? Like with my Witcher example, I could just imagine those games doing what the Mass Effect trilogy did, and included AI-controlled companions.

And, it's not that hard to write companion characters, either. When you look at Garrus, Wrex, Tali'Zorah, and Liara, they are technically members of a Five-Man Band, including the Lancer, the Big Guy, the Smart Guy, and the Chick, respectively. Using these familiar archetypes and building upon them, like BioWare did the Mass Effect trilogy, would not be so difficult when writing single-player campaigns for games like Watch_Dogs, Assassin's Creed, Call of Duty, etc.

EDIT: And not just the first Mass Effect, either. Remember the Suicide Mission in Mass Effect 2? Yeah, in my opinion, the Five-Man Band would have included the second team leader as the Lancer (e.g. Miranda, Jacob, and Garrus); the tech-expert as the Smart Guy (e.g. Mordin, Tali, Legion, and Kasumi); the biotic-expert as the Chick/Heart (e.g. Thane, Samara, and Jack); and the combat-experts who hold the line at the end as the Big Guys (e.g. Grunt and Zaeed).