They can't really go all that wrong with adding new defense mechanics, and by that I mean mechanics that simply don't work or feel like a waste of time, since they are mostly passive and add very little gameplay changes. Raiding mechanics on the other hand are much more complex and must need an amazing amount of planning, testing, braking and then re-planning.
The defense is a set 'weight', a set goal that people can reach for, there is very little variable in defending. Raiding is much more flexible and active and thus harder to define as it mostly depends on how far the attacker is willing to go. MM just needs to find the right combination of new things to get the right balance on risk and reward for both the attacker and the defender.