Funakoshi was not the only karate master going to japan (Motobu quickly comes to mind). And much of karates initial spread worldwide was due to US soldiers learning small parts of it while stationed on okinawa after the war. So Karate would still be known and spread.
But Funakoshi and Shotokan was undoubtly the most successful school, So I think that karate would not be nearly as large as it is now.
Also, without Shotokan dominating the field and influence almost all styles, I think it possible that karate would resemble its okinawan roots more closely.
Martin Hultgren
Kyokushinkai, Sweden