At the moment you can use a trick to play with your friends (that's what I'm doing): For each player, redirect m.agar.io to your own server (/etc/hosts), which serves a static file with the gameserver location (which you get from the original m.agar.io). Then reload the game until you're in the same realm. Don't forget to set the Access-Control-Allow-Origin header.

Could you go into more depth on how to do this? How would one force redirect m.agar.io to the server of your choice? Also, what is "Access-Control-Allow-Origin header"?

+1 for visible borders. How about power ups of some kind? Maybe a speed burst that shrinks you, for example?