athass_prkr
Even if it would be original if he was NOT registered for the tournament, we all know that the author will probably find a reason for him to participate. And in the event that Harry doesn't find a reason to enter himself (Arcturus might ask him to in order to improve his reputation on the international stage), there would be many potential culprits behind his forced entry. Dumbledore would do this because of their shared past in which he learned not to underestimate Harry and would see this as an opportunity to eliminate him. Voldemort should still focus on Neville, but he might consider entering Harry for his ritual. Grindelwald would be the least likely as he has had little contact with Harry and still shouldn't consider him a major threat. And then... there's Lily. Would she have any interest in doing this to her own son ? She seems cold enough to do that.
I personally will rule out Grindelwald and Voldemort for several reasons. First, let's start with Voldemort 1. There is a prophecy between Voldemort and Neville, so these two are connected in themselves, meaning if Voldemort believes that attacking Harry will help him return, then the matter is logical. He must target Neville because he is the child of the prophecy. We know that any prophecy that occurs in the world of magic has its power and mystery. Guys, you are only sufficient with the obsession that afflicted Dumbledore and Lily Potter in trying to break a prophecy or control a prophecy. This indicates the power of prophecies in the world of magic. As for Grindelwald, I think he is busy. For the upcoming war between him and Dumbledore, and even assuming that Grindelwald later intervened and tried to attack Harry, if this scenario happened, I believe that Grindelwald would attack Harry, not because of his strength or danger, but because of his heritage. Everyone knows that there is a special obsession with Grindelwald when it comes to the Deathly Hallows. For example, the Philosopher's Stone, the Invisibility Cloak, and the Great Wand are currently in Dumbledore's possession ( The high probability of the person who will intervene is 1. Lily Potter (Harry's mother) and then (Dumbledore) because of the conflicts and disputes that were between him and Harry previously. As for Grindelwald and Voldemort, they are the two people most likely to intervene, very weak. )