There are two types of card that will be supported to expand the functionality of ECLair, and all devices (serial ports, storage, etc.) will be implemented on these cards.
Expansion slots are for peripherals like serial ports, storage devices, etc. These will be similar in concept to ISA slots. Details about these will be worked out later, once the CPU itself is more finalized.
Extension slots are to extend the functionality of the processor itself. These will have the ability to access some of the existing microcode bits, as well as use of the bits that are available for future use. These slots will be used in combination with microcode extensions (via the writable control store) and could implement things like multipliers, barrel shifters, or even a full FPU.