I have just beaten FFVII: Crisis Core yesterday and it did lead to a definitive ending.
Zack Fair's fate was very certain that he would die at the hands of the company he worked for, but he took all of them down fighting and mortally wounded. This also explains why Cloud Strife had a personality disorder. Instead of mako poisoning, it was mako addiction. Cloud was pretty much an invalid while Zack was unaffected due to being infused with mako when he was with SOLDIER. SOLDIER was already breaking up near the end of the game, but it should have been disbanded by the Shinra Corp.. The biggest shock of all was Genesis living and being young again at the end of Crisis Core. I was shocked that Sephiroth clones (or Angeal clones) dressed in SOLDIER uniforms taking Genesis away in a Shinra helicopter. Two members of the Turks really do care for Zack: Tseng and Cissnei. I don't know about Cissnei's fate as this wasn't explained in Crisis Core. Tseng still serves with the Turks throughout the "Compilation of Final Fantasy VII". He was a nice person to Zack and they acted like friends.