Honestly, beyond a more in-game way to track the respawn timers for A and S (or at least being able to know if they're currently spawnable/alive), I don't think system needs much improvement. Blocking people from other worlds/DCs would make achievements for A and S rank basically lifelong commitments to achieve and nigh impossible for those who play in bad timezones; while invuln timers and such would just serve to congest servers further. The main issue ultimately, isn't that players are behaving 'wrong', or that world hopping is bad. Main issue is that there's not enough S and A marks for people to be able to grind for achievements and their cooldowns and random elements only serve to encourage both congesting and early pulling.