u can spectate ppl that r fighting... like for example u have a compass n when u right click it shows the ppl online [ not all ] or just tybe /spectate [ person's name ] then u will be spectating the person u want...
that would be interesting minus bow fights would no longer exist. As the plugin that does /spectate still has the player as an entity which an arrow dissolves in therefore some one would get their friends to form a spectating wall that would ruin alot of fights but it would be a good way of spotting hackers.
/spectator is a pretty nice idea... only the fact that noobs can just come up and become a "meatshield"... but there was one server where there's real spectating: you're not playing but you don't block any of the attacks. I don't know how they did that, but they did it and it works. No invisibility, but you're not seen by any other player.
If possible, do as Creepersteam had noted above. I am not going to repeat what had been said above, but I agree with it. If done I vouch for this idea.
