I myself love puzzling with reactor components trying to make nice designs. However since there only a few components there arent that many (good) reactors designs to make. In my list we have already found most if not all of them. Some new components would ad a new layer of depth to designing reactors. Could be improved components or components that are unique in their role.
So share your ideas about this :).
1 cooling cell
1 adv circuit
Cools at 1 heat per sec and in addition it generates 5 eu for every 1 heat cooled (this isnt op max eu output improvement is only 25% if everything is cooled by heat converters). This is more efficient at bigger reactors (because there is relative less base cooling).
1 gold block
4 adv circuit
Increases output by 100% and heat generated by 200%. Uranium is depleted only 75% faster (so this raises efficiency a bit too). For each chamber you need 1 more for this to function (so 1 chamber you need 2 and 6 chambers you need 7). New 'upgrade' slots need to be added to the reactor where you can put this in.