If you have a big island that can accommodate many people, consider using full benches. The benches can be with or without backing to allow for more comfortable seating.
Maximize natural light by having windows and skylights, and keep kitchen wall surfaces light in color to reflect daylight. Custom kitchen islands work great by using pendant or recessed fixtures to direct light onto the kitchen island and other work areas. Electrical codes will likely require that electrical outlets be located on the sides of fixed kitchen islands, not on the top, to prevent electrical shock.
In addition to seating 4 (or more), islands today are used to house "luxury" appliances that do not fit in the cabinet structure like a second microwave, second oven, wine refrigerator or even another cooktop. Islands many times are fully wired for electricity and have plumbing for a second sink. The base of an island can be used as a wine rack. The possibilities are almost endless.
Space and Design are Important : You need around 3 feet of walking space around the island unit, and you must be sure that the drawers and doors in the cabinets have room to open. If you keep to the 36 inches then you should be OK. The only dimension that must be stated is that of the counter height. Ideally it should be the same as the rest of your kitchen units. If not, then it is not a disaster, but can be disconcerting when you are working on the various surfaces at varying heights.