Yea, what I think is Reptile or any other host would be required for his complete resurrection. In the original timeline Reptile just chanced to be there at that time. This is the flow of events in the original timeline.
-Onaga's holy men captured Blaze to be the guardian of the egg and maybe even the host body for the egg once it hatches.
- Blaze however killed all of the holy men and escaped to continue with his quest , leaving the dragon egg unprotected.
-Nitara came to know that her realm's orb was inside the lava surrounding the egg. She abandoned Reptile for Cyrax as he could retrieve the orb. Once the orb was retrieved she left to her realm and Cyrax got teleported back to earthrealm. Reptile came in search of them to have his revenge for abandoning him and that was the moment when the Egg broke.
In the new timeline, the first and second events may most likely remain the same. However Onaga's holy men still can be able to subdue Blaze preventing him from escaping who could then end up being the host body for Onaga.
Also anyone close to the egg while it hatches can be the potential new body for Onaga. Also if I remember right, Onaga's true form is not how he appeared in deception. If you notice in Raiden's MKDA ending, Onaga looked much more intimidating than his current form. I think when Onaga's essense enters into a body, he transforms in a new body while keeping the traits of the host body (Reptile's scaly and green skin for example). If this is true then we may even see a different Onaga in the future depending upon his body he acquires for himself. Blaze and Onaga fusion anyone?