I tend to feel the same way about nationalism...i.e. the imaginary lines. I also see that same 'violation of individuals' being the case whether it's national or a world government...hence the need to something else.

I like the idea of each person being sovereign or choosing to buy in to some group system they align with. While it's not a 'dealbreaker of sorts,' when each person is sovereign, I have problems thinking of a way to prevent/solve conflicts beyond the age old 'might makes right' side of things. At least in the group sense, we'd be buying into to some set of rules that we would agree to agree on, and I'd assume some sort of repercussions (i.e. being kicked out) if they weren't adhered to. I'm not trying to argue your points here, merely where I hit a wall myself right now.

Short of 'trusting' everyone in the world to have that complete ownership (and hence responsibility) for themselves, I'd suspect some mechanism would have to deal with aggressive violence, theft, etc. since I'd see that becoming the norm, would drive many people to join a group out of fear/security needs. Idk, I'm just kinda thinking outloud at this point.

I've toyed with the idea of quasi-decentralized patterns for this for a while, where there are the larger groups with their smaller offshoots, but at least hold to some core, agreed upon concepts (like murder outside self defense is bad as a cheesy example) but then the smaller groups could have their differing 'local agreements' like whether marijuana was allowed or not (again just a cheesy example.)