For sharing bosses that'd be great, maybe planets too, but for player alerts? Think of all those people who alert for a single badge tap or for returning the favour... do you want ten legions hitting you at once because one person takes a badge tap personally?
Perhaps if there were an option in the diplomacy tab for two legions to formally declare war on each other, alerts to be shared with fully napped and battle napped allies... Heading toward a new object here, an alliance, federation, empire, whatever you want to call it, a legion of legions, imagine the bases we could make for one of those.
