Proposal - simply remove the ability for players to set standings to each other. This would not affect how standings with NPC factions work.
OK so as an idea this initially sounds pretty retarded, but think it through for a second.
The goal of this would be to nerf the fuck out of nullsec mega coalitions, meaning that coordinating with each other would be almost impossibly tough, with friendly-on-friendly killings happening constantly, there would be almost no way to keep the peace between two neighbouring allies trying to work together. Hopefully this would force people who want to actively work together into one alliance/corp, assuming there would be a natural limit to how big an alliance could get in terms of logistical and cultural difficulties/differences, nullsec would become a lot more fragmented, and frankly interesting. I think this would apply to lowsec and npc null too as well as sov null.
Obviously this would have a challenging impact on lots of people, in all aspects of the game, but personally I think it would really make being in an alliance more meaningful for non-nullsec residents, and hopefully be the shake-up nullsec needs to becoming more dynamic and interesting for small-medium alliances who aspire to be independent.