The tradeoff between element usage and other special items like the various nades and mines is not something I believe should be changed. It's balanced in such a way that you should choose between them. An additional degree of freedom was added when 2 elements could be worn.
However, the idea of resistance to the various items in the game sounds interesting. Not for snipers and shotguns or what not, because that kind of defeats the purpose and I'm pretty sure everyone would want to take less damage from shotguns, but resistance items just for the special items. I believe there actually were resistance items in the game for the elements, but those were removed (maybe the charms?).
Either way, the third item slot could include level 1-80+donor analogous for elemental resistance, explosion resistance, smoke resistance, flash duration, and others I may have forgotten. Level 1 might be -5% damage and the donor one be -30%. This way people will also complain less about all the heavy special item users in the busy TDM rooms.
If the third item slot is frowned upon, it can also be an infuse-able item onto an equipment item. However, we then should consider elemental buffs as well, which brings us to why charms were removed shortly after they were introduced in the first place. If this gets added again, it should probably only include resistances to special items specifically, be a tradeoff between the various resistances possible, and not completely negate the special item it resists.
I don't see much point in giving these no weight, It sounds fair that someone might have to use a weight ring instead if they want to take -30% damage from something. Plus, I believe with the standard 2 shotguns+sword+meds+cat in the hat, there is an excess amount of weight one can use for this new potential item.
