had things been different, and maybe kakashi had the ability to open all 8 gates? or even kisame? deff op w/o a doubt and deff waaay off topic, but how would that work out?
Well Kakashi would not benefit from the gates really because he has average chakra levels and with the sharingan already draining those average chakra levels, he would only be able to stay in the gates for a minute or two.
Kisame though, he would be a really good person to use the gates as he can heal the damage done to himself through Samehada and chakra, and he has a huge amount of chakra himself to be able to stay in the gates for a while.
Here are the people that I think could utilize the gates best, in order from best to worst (although worst just means of the ones listed)
Naruto - most chakra in the manga, and as the 9-tails jinchurikki he heals insanely fast. He could stay in the gates for an insane amount of time.
Killer Bee - same as Naruto except less chakra, and we have not seen if he heals at an insane rate like Naruto though he probably does.
Kisame - with his healing from Samehada, and his huge chakra levels and chakra stealing abilities, the gates would work well for him.
Tsunade - her ability to heal herself in combat + her large chakra levels would make the gates extremely useful for her. However she doesn't have as much chakra as the three above her.
Other people could use the gates as well, like the raikages, etc. but these people are the only ones who could heal themselves from the damage the gates does to the body.