I generally enjoy these invasion events. However, there are a few things I'd like to see happen:
1. The boss should have more health than he does now. The bosses go down so fast that it's often hard to get to it before it's dead. A lot of times, a single player is capable of taking out an invasion force. An increase in boss health would also promote killing off the cohorts before taking on the boss. Currently, most people DPS the boss until he dies (which is quickly), then take off for the next invasion mob without killing his cohorts.
Edit: And since arriving at Avril Volcano, I doubly recommend this. The entire invasion mob is mowed down in less than 2 seconds. It doesn't help things that the mob always spawns in the same place.
2. The icons on the minimap should update more frequently, or at least refresh when an invasion mob dies. I hate traveling a quarter way across the map to arrive at the scene and the boss has been dead so long his corpse isn't even there anymore. This also applies to guard crystals. I've gone to several red ones to find out that they're blue. Then I look at the minimap and it still displays as red. Super annoying.
3. The reward mails should only come after defeating the boss and the trash mobs he's bringing with him. Like I said above, people like to ignore the cohorts.
Also there's a desync bug that happens occasionally. A player will attack the boss mob, and the boss mob will take off running away from the attacker on my screen. Attempting to attack it will elicit a "The target is too far away" message, and I'm unable to attack it until it rubber bands back into place. So far every time this has happened, the boss was killed before it re-synced, resulting in no treasure for me because I couldn't attack it.
Edit: I also just got the bug where I wasn't awarded a chest for helping to kill one of these mobs. The first time out of many, many chests I did get, but still annoying.