One possibility is that Voldemort could initially be amused by Harry's request. He might toy with Harry, pretending to consider helping while actually having his own evil plans. For example, he could use Harry's vulnerability at that moment to gain some sort of advantage, like getting information about the Order of the Phoenix. Or, Voldemort could see it as an opportunity to corrupt Harry further, slowly turning him to the dark side by making false promises of power and protection.
The most obvious consequence would be that Voldemort would likely betray Harry. Voldemort is evil at his core, and he would probably use this opportunity to harm Harry or gain more power. For example, he could steal Harry's magic or use him as a pawn in a new scheme.
The Order of the Phoenix would be horrified. They have spent so much time and effort fighting against Voldemort. If they found out Harry had asked for his help, they would likely feel betrayed. They might even try to stop Harry, thinking he had been somehow influenced or corrupted.
Perhaps Harry discovers some dark secret about his own past or powers that he thinks only Voldemort can help him understand. For example, if he finds out that his connection to Voldemort is more complex than he thought and that Voldemort holds the key to unlocking or controlling certain aspects of his own magic.
Perhaps Harry has a plan to outwit Voldemort in the long run. By seeking help, he can get closer to Voldemort, understand his weaknesses better, and then use that knowledge against him. It's a risky but potentially effective strategy, like a spy infiltrating the enemy's camp.
At first, it would be extremely tense. Voldemort would be highly suspicious of Harry's motives. Harry, on the other hand, would be very cautious as he is approaching his mortal enemy. But as they start to interact more, there might be a grudging respect that develops if they find they have common goals.
In fanfiction, Dumbledore could have set up a series of trials or quests for Harry before the final battle. These experiences make Harry stronger both magically and mentally. Dumbledore might also have a secret plan that he has been preparing for years. He could be using his own powerful magic in a covert way to weaken Voldemort over time, like gradually breaking down the protection spells around Voldemort's Horcruxes so that Harry can find and destroy them more easily.
She might react with her usual no - nonsense attitude. McGonagall is known for being strict but fair. So she would first listen carefully to Harry's problem, her eyes fixed on him, and then start to analyze the situation. She may not show much emotion on the surface, but deep down, she's concerned for Harry's well - being.
She could also provide Harry with some protection. In fanfiction, she might be even more proactive in keeping Harry safe from the dark forces. For example, she could give him a magical charm or a secret spell that only she knows to fend off enemies. Or she could arrange for some trusted allies to keep an eye on Harry when she can't be around.
One possibility is that Harry has discovered some long - lost connection between them that he thinks can be exploited. Maybe he's found out that Voldemort has some knowledge or ability that is crucial to solving a problem. For instance, if there's a prophecy that only Voldemort can decipher, Harry might approach him for help, despite their long - standing enmity.