They just have to be interesting. That goes for both their story AND game play. The original version of DMC3 only had Dante, and it's still one of the greatest games of all time.
However Dante is interesting, and surrounded by interesting characters. His game play also has a truly mind-blowing amount of variety, to the point where as long as you're willing to experiment with weapon/style combinations you will find more than most people could ever want.
HOWEVER.
DMC3 is immediately supplanted by DMC3SE, which adds Vergil and thus the game is immediately improved upon. DMC4 adds Nero, and then is itself supplanted by DMC4SE which adds Vergil and Lady.
I would be initially disappointed if I heard FF16 only had one protagonist, but again if they show game play and story that is fundamentally interesting then I'd at least give it a chance. However every interesting character they show around them would always trigger that "what if..." Response.
Looking back on FF15 as it launched, Noctis may have some interesting stuff but he just doesn't have enough to carry through the entire game. FF15 as it is today, with all party members playable is (in a more ideal world) how the game should have launched.