How does Hisoka see Gon exactly? As a rival or as a toy to entertain himself? Hisoka hinted that Gon is his treasure and also his prey, but it can't be both, right? Because in the end, he didn't want Gon to die no matter what.

So it's weird that he didn't want Gon to die but also made him his prey to fight till death. Is it sexual for Hisoka or what? I still remember the 'Schwing' thing.
Hisoka is a psycho that only seek for strong opponents to fight.

He saw a real potential in Gon since they met (Killua too , Leorio and Kurapika too) and just want him not to die until he becomes a monster worth to confront.

And yes , for psychos , the pleasure they find in their... "activities" are orgasms-like.

Remember just what Hisoka had to do only to "fight" Kuroro , he likes to take his time but in the same time when he fought Gon he could hardly prevent himself from killing him ^^

So we can't say he's a rival , he's just another "toy" for Hisoka , but a special one as he really has the potential to become a monster.