Any change that includes the structure component would need to have either A) structures be so essential to EVE gameplay that even the smallest one-person corporations would be shooting themselves in the foot not to have one (in their own corp, rather than an alt or public one), or B) they still include some means to declare war (if a more limited one) on structure-less people.
No one in EVE should be able to expect perfect immunity.
Really, though, the problem with most structure-based proposals is that they come at the problem from the wrong angle. The issue has never been in how wars were declared (except the inability to war dec people in NPC corps, and maybe in how many corps you could dec simultaneously), but in how wars could be fought and ended (or rather, the lack thereof). Valid counterplay is what was lacking. Players log off/dock and stay logged off/docked because blueballing until the deccer grows bored and ends the war themselves was the only real “tactic” that people had to end an undesired war.