1st off, I'd like to say that i realize everything I say could be dabated, or explained, and I acknowledge that, so don't feel the need to do so. Thsi is merely me nitpicking at soem thinsg I feel were oversights. I know that it's no big deal, since they can just pick and choose selective elements from these endings for the next game, as always, so please don't feel the need to try to "prove me wrong." I am not trying to prove or disprove anything, I'm merely offering up some thoughts.
That being said, potential spoilers follow...
Now, In Sindel's ending, among other sources, we learn that Onaga is indeed hijacking Reptile's body.
In Havik's ending, upon eating Onaga's heart, he gains Onaga's fabulous powers of ressurection.
Now, taking these two events into account, should Havik not have been merely eating a suped up Reptile heart? Onaga was dead upon defeat. His soul should theoretically have moved on to someplace like the netherealm, or back to the void, upon death, thus leaving a dead reptile body lying there, as in Sindel's ending.
She ripped his soul out, and he became Reptile again. We've learned that in MK, souls move on after death, be it to the heavens, the netherealm, into Shang Tsung, wherever, they don't stay in the body.
How is it this body, which was not Onaga's, retained his form and his powers upon death in one ending, but not in another?
And the only reason I bring up this seemingly minor nitpick is that, this could put a big old crimp in the validity of Havik's ending, which, as we all know, features the return of Shao Kahn.
But, as I said, it could easily be fixed should they need it to be. We must wait and see if we shall soon be witness to the emperor's return.