The events where we find special enemies seem very sparse at this point in time. In all the time I've spent playing missions, I've only encountered two events. And it'd be a cool idea to have different, "larger" raids in explore missions since the player density is increased. Then again, they may already exist. I just don't seem to find any often whatsoever. And these bosses, event or not, barely drop anything. Over the time I've played, that is.
I think they should be a little more common and more verity to them, also maybe a chance for a harder one based on number of guardians in an area.