I actually don't think that damage reduction mods for dropsuits, especially specific type (Blaster, Projectile, Laser, etc), is that useful or even needed in the game. Because every suit has a varying level of base HP, it would be nearly worthless of some and barely useful on others. Scouts, for example, with their 200 Armor get next to no benefit from saying "I reduce Projectile damage by 10%." A Militia Shield Extender is more helpful.

Sentinels are the extreme of course. An Amarr Sentinel can get 1,344 armor with 4 Prototype plates (not unheard of for basic defense with Logi support). Every plate is going to add 148.5 HP. Having 3 plates would be 1,195.5 armor. Well, 148.5 / 1,195.5 = %12.42. This means that to be even worth it at the highest level areas, the resistances would have to be at least 13%. You have to make the number big enough to not just allow base HP to be superior but small enough that Sentinels cannot become even more powerful. There is pretty much no way that it is going to be less used armor plates for any non-sentinel.

For the Equipment thing, I could see something like that being implemented for Assaults. Make some Equipment that reduces damage by a certain percentage but has a long cooldown and has a penalty after it turns off with some kind of CD afterwards. IE, activate to take 25% reduced damage/increase damage done for 20 seconds but for 5 seconds afterwards your speed is greatly reduced/you cannot move/you cannot fire your weapon. Give Assaults an extra Equipment slot for it, like happened to Scouts.

It makes the Assaults the guys that are there to break in an enemy's location when they have Sentinels and Logistics and makes the Sentinels more based on defense and not breaking into an area that is really locked down.