Lavos could do it if he is sentient and willing to become the Time Devourer. But that were just examples anyway, don't take it too seriously. But I still think that another being might be the entity. On another theory, the entity could have being the characters. Let me explain it, all of us wish to in some way or another, 'save the planet', we live in our own universe. So, maybe one of the 'heros' might be the entity, and this person may even not know this.
Again, just examples.