First of all, it was fifty years ago that Netero was regarded as the strongest in the world, top tiers like Ging and Chrollo weren't even born back then and probably most if not all of the top 5 nen users weren't born yet. Netero was definitely not the strongest human when he fought the King. Also, it's obvious that the monsters in the dark continent will be stronger than Mereum. Think about it, they sent two rookie kids, an old man past his prime, two average warriors (Knuckle and Shoot), and two above average/strong ones (Morel and Knov) to deal with the chimera ants. Comparatively speaking, look how many warriors are going to the Dark Continent. We have the princes all bringing fifteen bodyguards each, and then they too seem haxed with their nen beasts. Then we have the zodiacs along with Pariston and Ging. We also have Beyond Netero and his crew. It's obvious that what awaits in Dark Continent is far greater than the King of the ants and his army, hence why so many strong warriors are involved this time around, some of which are stronger than Netero if you follow logic and what was said in the manga. Lastly, regarding Gon going to the Dark Continent. I don't see why not. Killua is already strong enough to go in my opinion thanks to Kanmaru, he can pretty much evade most attacks launched at him and he's an assassin so he can deliver the killing blow. Gon I think just received a new idea for his next hatsu/ability from his transformation. We've seen many enhancers manipulate their body in various ways, examples are Youpi who can transform/shape-shift his body into various convenient shapes and sizes, and we have Hollow's (the owl chimera ant Gon fought) Gorilla Mode where his shifts/grows/tranforms his body into a much bigger, durable, strong, and more muscular form for combat. I don't think it's a coincidence that Gon fought Hollow and briefly encountered Youpi before he made that Gon-san tranformation. I think that if he can increase his aura reserve enough he can do something similar again (we've already seen two enhancers do it without a vow/restriction). It will require alot of aura though so he needs to train hard to build his aura reserve and do it, and even then, I don't think he'll be able to do it to the same extent, maybe he'll be able to turn himself into a muscular sixteen/eighteen year old at the expense of a great deal of aura and with a time limit. Special mention to non-enhancers/manipulators who can manipulate/shift their bodies (Neferpitou, Bisky). Netero also did something similar although a smaller scale (grew his muscles in a way that they sealed his wounds) and even Killua does it to turn his hands into claws. I apologise for the wall of text but my phone is screwed up that way, it doesn't use spacing on this website, anyway, I think Gon is set for his next power-up, I could be wrong though.