As an arcology is a huge structure, the building itself is of monumental importance. An entire community resides in a single structure, and a failure of that structure would be catastrophic for the community.

As such, the structure should be as strong as it is humanly possible to build. It should be able to withstand any number of possible catastrophies, natural or otherwise.

Another aspect of the structure I believe is important is that it should be built to be expandable. The sides of the structure should be designed so as to accomodate another arco of similar construction being built alongside it. There should be some space in between so as to isolate any problems one arco structure may experience (fire, epidemic illness), but they should be close enough so as to connect them structurally, strengthening each building by making it essentially one even more gigantic structure, and close enough to easily connect them by walkways and light vehicle transportation (people movers such as golf carts and cargo carriers similar to forklifts), so that residents do not have to return to ground level merely to go from one building to the next. The essentially makes the arco as it is built merely "Stage 1" of a much larger project, expendable by building similar structures as many times as may be needed to accomodate the number of people and businesses moving in.

As the structure is supposed to be extremely tough and also expandable and as such views may be blocked by new construction, the outsides of the structure would probably not have windows. However, people can't live without sunlight forever, so at the top of the arco there should be some sort of recreational facility to accomodate human desire and need for time in the sunlight. This may be over a hundred levels above ground, so it would probably need to be covered to protect people from wind and cold at that height, but it is a vital part of healthy living in what is otherwise a lifetime spent indoors.

Based on the amount of weight various industries can put down on a floor, as well as proximity to outside road and rail transportation, any industry should be at the bottom levels. The top levels just below the "rec deck" roof should be residential and a barrier of commercial and office space separating the two. The order of separation is an idea I got from Sim City I must say, but it makes sense.