Multiblock versions of certain IC2 machines, probably basics like furnace/macerator/compressor/etc., with upgraded functionality similar to how the EU reactor inside the pressure vessel becomes the fluid reactor. Kind of like the ol' Advanced Machines mod, but instead of crafting a new machine you'd have it inside a 3x3x3 shell or something.
The reason I suggest this is, well - I remember playing a version of GT a long time ago, probably GT2/GT4, and was so impressed by the multiblock machinery there (which if I remember was various 'machine blocks' with a single machine that would detect the blocks it was attached to, no updating or combined textures or whatever). Thematically, like - it /felt/ industrial. It felt big and impressive, like you'd expect a factory machine to look and act. It'd be really neat to have something like that in core IC2, and if the mechanics are effectively already there in the form of the fluid reactor - I/O ports, redstone control - it just makes sense to me.
I love the fluid reactor btw. <3