Now, you may be thinking i'm crazy, but I can support this. Many people of the Harry Potter fandom have asked "why make the Triwizard cup the portkey to take Harry to Voldemort?" and I think this theory will answer that.
Voldemort intended to make Harry win the tournament, have the portkey take Harry to him, get all resurrected, then kill Harry, and take a Polyjuice Potion using Harry's hair to sneak into Hogwarts, and kill Dumbledore when he wasn't looking.
My evidence: First, think of all the portkeys ever mentioned in the Harry Potter books. The only one that was two-way (it would not only take you to a place, but also bring you back to where you left from) was the Triwizard cup. I dare you to name a portkey in the books that is also two way. You can't (Mr. Weasley saying "get back to the portkey" in the 4th book doesn't count, since it is never established that it would have been the same portkey, and the portkey they used to get to the world cup lost its enchantment when they landed the first time). I see this as a way for Voldemort to return to Hogwarts, and believe that it was made a two-way portkey on purpose.
Next, why give harry his wand? Voldemort is newly resurrected from being "less than the meanest ghost", and is facing the one who caused his almost-death. Why would he give that person their wand? Why duel them? He may think he figured out why he was almost-killed, but surely he had some doubts? And his Death Eaters would still remain loyal to him if he had just killed Harry while he was tied up. I believe that he dueled harry to win his wand from him, so that he could complete his disguise for when he returned to Hogwarts. We know from "Harry Potter and the Deathly Hallows" that he believes that, in order to "win" a wand from someone, they have to be beaten, and in order to beat them, they have to be killed (this thinking is what led Voldemort to kill Snape). So he dueled Harry to kill him (a bonus because prophecy) and get his wand, which would then change its allegiance to Voldemort, and get Harry's hair, so he could go back to Hogwarts as Harry, and kill Dumbledore.
Now, why choose the Triwizard tournament, and the Triwiazard cup to achieve this? Well, if Harry wins it, he would have a mix of emotions, so it would seem normal if he seems different upon return, so no one would question the changes with Voldemort as harry.