I think there were things they TRIED to keep secret in MK9 & MKX, but indeed the games did get hacked to death, and indeed most answers were out there (in an unofficial capacity) before final release.
The only thing I remember staying relatively secret in MK9 were the hidden fights with Klassic Reptile, Smoke, Noob & Jade. The achievements were out there, so people had an inkling to look for them, but no one (at first) knew quite what to make of them or how to unlock them. I recall there was a big long thread where we shared tips and stories of what we were doing when they appeared, and how we might unlock them.
There may also have been some alt costumes in MK9 that stayed hidden before release, but who remembers.
The biggest thing I remember remaining unconfirmed before MKX hit release was honestly the final boss. They never actually confirmed the final boss for MKX. All signs pointed to Shinnok, but NRS played coy about it, leading some folks to suspect there might be a twist boss. And there kinda was, I think they were just trying to keep Demon Shinnok under wraps. Though I remember people speculating more out there theories like Reiko or Havik or Onaga before release (mostly fueled by the comic book). Of course it didn't take long to get the answer upon release. And since a few copies leaked a couple days early here and there, we more or less had the answer before the game ever reached most of our hands.
ANYWAY yeah... I hope if those 3 are secret characters they stay as buried as possible prior to release.
Honestly the way they've kept Frost hidden in plain sight in the new trailer, feels like they're trying to keep a lid on her role in the game as well. We also haven't seen much of Noob or Jax, despite the leaks seemingly confirming them. Not sure if that's because they're significant, or just because they got developed last.
