Had Lord Voldemort made Neville Longbottom the Chosen One, could Neville have vanquished the Dark Lord instead of Harry Potter?
Absolutely. The son of Frank and Alice Longbottom is almost always underestimated, yet other than the book series namesake, Neville grew most, both in spirit and ability.
“…You know Voldemort’s snake, Neville? He’s got a huge snake… Calls it Nagini… …It’s got to be killed…” (Harry Potter) … (Albus) Dumbledore had died knowing that three people still knew about the Horcruxes; now Neville would take Harry’s place… “Kill the snake?” (Neville) “Kill the snake,” Harry repeated. Harry Potter & the Deathly Hallows - J.K. Rowling
Like a dying Knight bequeathing the defense of the Holy Grail to the next Guardian, Harry chose Neville to replace him within his trio of Dark Wizard Hunters, when it became apparent young Potter had to sacrifice himself to Voldemort.
The instant response might be to suggest a loose application of poetic license, yet to consider anyone else to inherit that deadly position would be to concede to an inferior choice.
Neville, time and time again, proved himself resilient and capable.
When the Death Eaters began to escape from Azkaban, in the midst of Harry’s training of Dumbledore’s Army, it was Neville who progressed the furthest with his Patronus and defensive spells.
The manner in which Neville’s parents were mystically brutalized by Rodolphus and Bellatrix Lestrange would have forever quelled the spirit of weaker souls than young Longbottom.
Nevertheless, it served only to steel Neville’s dogged, courageous character.
“My son and his wife,” she (Augusta Longbottom) said… “were tortured to insanity by You-Know-Who’s followers… …They were Aurors… very well respected within the Wizarding community…” Neville looked around at the others, (Harry, Ron Weasley, Hermione Granger, Ginny Weasley) his expression defiant, as though daring them to laugh… Harry Potter & the Order of the Phoenix - Rowling
It is misinterpreted that as a student at Hogwarts School of Witchcraft & Wizardry, Neville was less than capable, for the future Professor of Herbology more than held his own in the Battle of the Department of Mysteries, the Battle of the Lightning Struck Tower, and the Battle of Hogwarts.
Most remember only that Neville ended up with a fat lip in the skirmish which led to the departure of Sirius Black behind the Veil.
It’s somewhat unfortunate that Neville seemed so often to serve as comic relief, for it makes it that much easier to forget his critical contributions.
Remember, it was Neville, in conjunction with Ginny Weasley, who would organize the DA to rebellion and continue preparations for the eventual showdown between the defenders of Hogwarts and the Dark Lord’s Death Eaters while Harry, Ron, and Hermione were on the run and hunting Horcruxes.
While it is likely even the late Professor Dumbledore recognized something greater in Neville, it would be the strict, but kindly Minerva McGonagall who would give Neville the boost of confidence every kid who wants to develop determination and courage receives from that one positive influence who empowers them to follow their hearts.
…snorted Professor McGonagall. “It’s high time your grandmother learned to be proud of the grandson she’s got, rather than the one she thinks she ought to have – particularly after what happened at the Ministry.” Neville turned very pink… Professor McGonagall had never paid him a compliment before… Harry Potter & the Half-Blood Prince - Rowling
When we reflect on this question which can never be truly answered, what perhaps actually resides beneath the surface is a question as to Harry Potter’s own abilities, and how Harry’s survival hinged more so on the actions of others than his own magical fortitude.
It is well-documented that Harry Potter was far more lucky than good.
The list of child-wizards more capable than the Boy Who Lived is not short.
Arguments could be made about nearly all of the Weasley kids having more talent than Harry; Hermione Granger needs little support to make her case; even Harry’s nemesis, Draco Malfoy, repaired a Vanishing Cabinet and disarmed Albus Dumbledore on his own.
Understanding that this question really refers to what might have occurred if Voldemort had made Neville the focal point of the Lost Prophecy, could Neville Longbottom have conquered the Dark Lord, had he been in Harry’s shoes?
We think so.
Of course, any guess would be the definition of conjecture. But this much might be stated:
If Harry had died, and might Voldemort have lived, it would have been Neville Longbottom who stood the best chance of defeating the former Tom Riddle.
Not Ron Weasley; not Hermione Granger; not even Kingsley Shacklebolt.
More so than anyone else, from the day they first attended Hogwarts, to the moment Harry met Albus at King’s Cross, Neville mirrored Harry's feelings toward Voldemort.
As much as Harry Potter would fight to the last against Lord Voldemort’s horror, so did Neville take every opportunity to defy the Dark Lord, even to Voldemort’s face.
Lest we forget, the last Horcrux, Nagini, was destroyed by good Mr. Longbottom.
The scenarios are too many and the possibilities so endless that we risk falling into the vortex of the impossible question here.
So, let’s at least say that if anyone other than Harry Potter might have stood a chance against Voldemort - whether in the beginning, had Voldemort chosen a different one, or in the end, were Harry to have followed Dumbledore into the light at King's Cross - it would have been no one less than Neville Longbottom.