Currently there are no disadvantages to invading someone. Yeah, if you kill an invader they have to go an rescue a bloodstain but generally this is only ever a minor inconvenience. Whereas if you die then you lose your humanity. There should be a penalty for being a losing invader e.g. it costs a set number of humanity to invade and you lose all of it if you fail or maybe the host getting all your souls.
I actually think in future souls games it should go even further. Remove invading altogether (except Darkmoon invasions). Instead implement a co-op system where you can be summoned, help the host kill the boss and then give the summoned two options at the end of the fight. You can either split the souls with the host or duel them for all the souls (the host would still get the items). Then Darkmoonies could invade the guilty and for various rewards. I feel this system fits in better with Dark Souls than the current one which is more reminiscent of bullying.
I'd be interested to know what you guys think of this!