In all other cases except the Triwizard cup, portkeys only go one way at one specific time. Touching them again does not activate them to return to their place of origin. Also, when Harry grabs the cup a second time, it does not return him to the middle of the maze. It takes him to the entrance of the maze, in front of everyone.
Therefore, when Crouch Jr. (as Moody) bewitched the cup, he planned to have it take anyone who touched it first to the graveyard, then to the front of the maze.The cup was probably supposed to be a portkey to take the winner to the front of the maze anyway, so they wouldn’t have to try to fight their way out again.
Voldemort obviously planned to kill Harry. He had to. That was the whole point; to kill Harry in front of all his Death Eaters, all the ones who had deserted him and doubted his power to return.
There’s the possibility that he wanted to send Harry’s body back, either to divert suspicion somehow or to intentionally flout his victory in Dumbledore’s face. Except Voldemort had promised his precious Nagini several times she could eat Harry, and it seemed like a promise Voldemort was going to keep.
So who was meant to take that return trip?
Voldemort could use it as a ticket into Hogwarts for a surprise attack, but he’s freshly reborn, his Death Eaters are 13 years out of practice, and there’s a flock of powerful wizards there for the Triwizard. That would be an idiotic move.
Or what if Harry—or someone who looked like him—had returned to Hogwarts as if nothing had happened in that maze? As the victor of the Triwizard Tournament AND the Boy Who Lived, Harry would be able to go anywhere and do anything. Everyone trusts him.
Two words: POLYJUICE POTION.
There was one Death Eater already waiting at Hogwarts who had very carefully been spending a whole year getting to know Harry, watching his every movement: Barty Crouch Jr.
So here was Voldemort’s complete plan: Use Barty Crouch Jr. to infiltrate Hogwarts as Moody. He gets to know Harry and sets him up to be selected for and eventually to win the Triwizard Tournament. He makes sure Harry touches the cup first. Harry is then transported to the graveyard where Voldemort is waiting. Voldemort uses Harry to rise, calls his Death Eaters to him, and then humiliates and kills the Boy Who Lived in front of them.
Then Voldemort strips Harry’s body, takes his hair, and transforms into him (or else has one of his DE’s do this—but really, who would he pick? Lucius is an idiot, Bellatrix is still in jail, and he believes Snape has deserted him). He then takes the cup and goes to Hogwarts as Harry. Later that night, Moody disappears, and Crouch takes Voldemort’s place as Harry Potter. Then, when the moment is right, Voldemort-Harry or Crouch-Harry will assassinate Dumbledore (incidentally gaining the power of the Elder Wand, though he wouldn’t know it), stage a coup of Hogwarts, and take over the wizarding world.
Heck, he/they might not even drop their disguise as Harry. The wizarding world has faced Voldemort as an enemy before, but if their savior Harry Potter suddenly turned out to be just as powerful a Dark Lord as He Who Must Not Be Named? It would be a far scarier prospect than simply dealing with Voldemort’s return.
It solves the problem of why Voldemort went to such lengths to get Harry through the Triwizard, when there were far easier ways to capture him: Voldemort didn’t just need Harry’s blood; he needed Harry as the world’s hero.
And all that time in Hogwarts would give Voldemort time to search for a relic of Godric Gryffindor, the one founder he never made a horcrux from.
Of course, none of this could have worked because Voldemort could never in a million years fool Ron or Hermione or Dumbledore, not even for a minute. But there’s Voldemort’s greatest weakness again—he doesn’t understand love.