A "defense" body part
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I think one type of body part that hasn't been covered yet is a DEFEND part, which would represent a creep's ability to dodge/block attacks. This part would actually decrease the amount of damage inflicted upon the creep. A regular TOUGH part, on the other hand, increases the creep's hits but doesn't decrease the amount of damage the creep receives. The DEFEND part would thus add a lot more protection than a TOUGH part, but at a higher cost.
The advantage of this part would be the creation of creeps that are able to withstand more attacks, but which are able to move faster than a creep weighed down by a bunch of TOUGH parts. A creep with a DEFEND part would also require less healing, as it would have less damage to be healed.
I envision creating a healer bot that is quick enough to move to where it's needed, but also able to dodge some enemy fire while healing other creeps. There would also be more protection for transport bots, which can't afford to be weighed down with a lot of TOUGH parts if they already have a lot of CARRY parts.
The DEFEND part would probably require an extension. I don't know how much they would cost.
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Sorry, don't want to hijack your suggestion, but I have a similar suggestion, which is not exactly same but, since it could offer something towards your own goals, I am posting it here.
Imagine this amazing announcement.
We have added a new body part - SHIELD - equip your creeps with this new and extra shiny body part to protect your screeps from harm. SHIELD will absorb all hits on the body part, but will use up 1 energy for the X damage absorbed. Your screep will also need a CARRY body part to keep its shield charged,
But wait, this is not all - the new amazing body part from the Spawn Werks(TM) also includes an ability to prolong life of your creep by utilizing advanced gene-editing techniques. It won't keep your creep alive forever, but will prolong its life per rate of Y energy per tick.
Imagine the siege battles, with tankers and fast supply tenders, defense of the supply lines and daring raids into the enemy territory to choke off the flow of energy.