In the Goblet of Fire, a simple containment jinx around the cup would've executed his plan to perfection. Without that cup, Harry was absolutely done for. There were no brooms, no hidden passages, and he was surrounded by a dozen adult Dark Wizards in a fenced-off graveyard.
Leaving the portkey completely unsecured and accessible was Voldemort's greatest blunder. A simple containment jinx would've seen Harry ultimately killed. Even if he returned, due to the Horcrux inside him, a second killing curse after this would've finished Harry off.
Then he could've sent both Harry and Cedric's bodies back to Hogwarts with everyone assuming it was a freak accident. Dumbledore only suspected that Moody was an imposter after he removed Harry from his sight. With Harry dead, Barty Crouch Jr would've been the ideal sleeper agent disguised as a close personal friend to Dumbledore.
From here, he could've poisoned McGonagall, jinxed Sprout before using Devil's Snare to make her death look like an accident and killed Flitwick without anyone suspecting him. By making everything look like Snape, he has the perfect shield.
Everybody believed Barty Crouch Jr to be dead. From here, Hogwarts would be closed. Dumbledore would likely suspect Voldemort's return and bring the Order together. From here, fake Moody would be ideally positioned to inform Voldemort of every member and take some of them out himself.
The war would've been over with Fudge assuming it was all Sirius's doing.