This can be done in a variety of ways, and even after this, some people - including myself - will keep using their regular class or glass-cannon sets.
1。 Defensive armor sets
Method #1: Adding one more status to resist.
Currently, the only armor set that has resistance to 3 statuses is the divine set so I suggest he following:
Royal Jelly set: sleep, stun, poison
Grey Owlite set: fire, freeze, shock
Dread Skelly set: curse, freeze, poison
They will each have 4 points of resist per piece which is the equivalent of a max unique variant.
Method #2: Adding more specialized defense.
All of the 3 specialized elemental defenses should have an increase, I suggest it being their normal defense bar plus the equivalent of a max unique variant. It appears to be as much as the equivalent to a high unique variant at the moment, I think.
2。 Monster-specific sets (also applies for divine veil and radiant silvermail)
I suggest adding a low on all of them. Sets that focus on only one monster family will have it at high instead of medium per piece which is max instead of very high for the whole set. Sets that focus on two monster families will have it at medium instead of low per piece which is very high instead of medium for the whole set. As for the fallen set to be balanced after this buff it will have its damage versus fiend penalty decreased from medium to low per piece which is medium instead of very high for the whole set.
I think another way you could approach this is by making it so that armour affects the damage you take more on higher difficulties. So on elite, it'd be practically impossible to survive more than 2-3 hits with glass cannon sets, but if you have the right defense (e.g. skelly set in FSC) you'd take FAR less damage.
Not only would this encourage people to expand their arsenal, it'd also work to nerf glass cannon sets, because they're 100% the best armour in the game. This could change it.
You could also do that in addition to your suggestions, since specialised sets are very underpowered right now.