So basically, to prevent really strong nations from wiping out small nations easily, a new rule is put into place. A nation cannot attack another nation that is more than 1 tier below them. Ex: S-tier nations cannot attack B-tier nations. This is only the case for attacking nations. You can attack a nation any amount of tiers above you, though you will probably lose. The only times you can invade a nation that is 2+ tiers below you is if you are drug into the fight because of alliances or treaties that were signed before the war. No trying to misuse this rule or you will be punished with a strike and a void. Normal breakage of this rule is a void, with repeated breaking being a strike and a void. TLDR: No attacking nations that are 2+ tiers below you unless they attack you or your ally first. This allows new-gens+casuals some room to breathe and expand.
The tier list to the left is an example tier list of what could be used, with Space Nations not being able to invade anyone with the blockade of Sol. This tier list may not be completely accurate, but it is a rough estimate. Potential changes: make it got from 1-10, and not be able to attack someone more than 2 below you.