Most of us seem to be envisioning some sort of broader structure to fit atop the spar design. It's probably going to be impossible to build a platform and then just hoist it in one piece to the top of the spar. So first, some criticisms:

• It's got to be assembled after the spar is vertical, which means at sea.
• This means another platform from which to work while this is happening, probably a tender ship of some kind, perhaps the one that towed to spar into position in the first place.
• The interior of the spar won't be completed either, until it's vertical.
• Just too many things that can't be nailed firmly into place while hanging in mid-air.
• I can see it quite likely taking months to finish the Spar simply to sustainably live-able standards after towing it out to sea. So much of this comes from the need to re-orient the structure once in place, and that's due to the deep draft.

Some of the design requirements will depend on the size of the spar,  what you plan to use it for etc. But here are some ideas based on some requirements I have gleaned from discussion:

• Either the top of the spar or the platform needs to incorporate a safety hull
• The platform should be designed with occasional extremely windy conditions in mind
• Square gives more surface area, but round(ish) probably balances better, it's better in high winds, and may be easier to construct: leading in to the next point.
• A conical safety hull attached to the spar and leading out to the edge of the round platform gives a continuous buttress to the outer edge of the platform, as well as providing useful enclosed cubic for living/working/equipment. In fact, properly designed trusses could extend quite a way beyond this buttress.
• This conical safety hull could be integral, or bolted on. Either way, it requires to ability to withstand a fairly harsh drop to the water's surface. It doesn't have to be a continuous hull, it could be seperate sections.
• The bottom of this cone could probably be down to nearly the average wave height. We wouldn't worry as much about rogue waves and high seas, because they're rare enough and the increased surface area presented to the wave is gradually increasing in inverse ratio to the probable height/force of the wave.
• It eliminates or minimizes the need for suspension cables cluttering up the overhead, which will be busy enough with radar/radio masts, windmills, flight deck, etc. 
• Trusses for decking from the outside edge of such a conical safety hull to the spar hull are fairly simple to design. Leaving the center of the Spar open for elevator shafts etc.
• The enclosed (lower) decks (of the platform) could be pre-fab/modular wall sections like the Deltec Homes design (tho almost certainly made of metal).
• However, the trusses that they use for flooring would probably be wall-height for at least some of the pie-sections.
• The more pre-fab and modular things are, the more of the work can be done on land. This is most of all SAFER, but also cheaper, and faster.
• The top deck would have discrete structures on it, geodesic domes built from modular panels where purposes of the structure don't actually negate the possibility of domes, as well as windmills, equipment masts, cranes, hoists, flight deck etc.