There was always the chance that the Triwizard tournament ended up being held at Durmstrang Institute or Beauxbatons Academy of Magic. If this were the case, then since Neville is under seventeen, he would most likely not left Hogwarts. Voldemort would have had to settle for the blood of an ordinary enemy.
Sexy David Tennant enters Neville into the Triwizard tournament. (The Doctor makes a second appearance!) Perhaps Ron did not believe Harry did not enter himself, but Creager and I believe that if Neville says he didn't enter, Harry and Ron would have to believe him. This avoids a great deal of high-school drama, and Ron himself tells Neville about the dragons. If Neville does not tell Cedric, then Barty Crouch Jr finds some other way of helping Neville with the egg. Neville is an altogether good person though, and would inform Cedric of the dragons.
If Neville had the misfortune of drawing the Hungarian Horntail to battle, we must report that he would fail and die. DEAD. Not being very gifted in the broomstick department, Neville would most likely me fatally wounded by the other three species of dragon as well. DEAD. If Neville and Harry asked James for advice, he would no doubt tell Neville what Sirius tried to tell Harry in Rowling's series—aim for the eyes. Hopefully Neville would survive the test and be awarded the golden egg.
Cedric would return the favor to Neville about the dragon and tip Neville about the prefect's bathroom. Since Neville would have met Moaning Myrtle, she would guide him in the correct direction to submerge the egg. If she did not, then no doubt Harry, Ron, and Hermione thinking together with Neville would result in hearing the song. Neville is very fond of herbology, and would automatically turn to gilliweed. Hermione would have taught Neville the stunning spell, so he would have made it past the grindilows and saved his 'loved one', most likely Harry. Hopefully Neville wouldn't be so gullible as Harry to wait for the other contestants, but either way would reach the surface.
Now Neville gets to enter the evil shrubbery of doom [dramatic music]! Barty Crouch Jr would mostly clear obstacles out of his way, but would have to leave the sphinx. He could chose to backtrack, but may get lost and Crouch would have to lead him another way. He could opt to solve the question, but there is a chance he could get it wrong and be mauled. DEAD. We assume that Neville answers correctly and moves on. Then Neville happens upon Cedric. If Neville sends up red sparks instead of actually saving him, then Cedric lives. Most likely Neville would save Cedric, and they end up grabbing the trophy together.
Neville and the revived Dark Lord are dueling. Sadly, there is always a chance that Neville authentically is killed. DEAD. Hopefully he does cross curses with Voldemort, and Neville watches Voldemort's wand throw up. Neville's parents or grandmother would tell Neville to release the phenomenon. It is possible that Neville would not think to summon the trophy, so there is a chance he would be hit by a spell. DEAD. There is also a chance Neville would not think to clutch Cedric's body before touching the trophy. This would complicate things quite a bit. There is the issue of reaction time of the crowd upon Neville's return. Without a dead body to confirm his words, things would have taken longer to kick into action. Either way, Barty Crouch Jr would go out of his way to get Neville alone and question him about the Dark Lord's revival. Dumbledore would still catch Barty Crouch Jr, and all would end as it did in Rowling's novel.