Insulation
I wonder what feasible changes could really be made to make new copies of the original twin towers safer.
Be sure to deal with the fire insulation issue. Whatever they replaced the original asbestos with when environmentalists forced them to alter the specifications, didn't get the job done.
To be fair, given the huge fuel-fed fire, there is no proof that an asbestos insulation would have completely prevented the collapse, and even had the towers held out for another hour, I'm not sure any more people would have been saved. I think a lot of the tenants that were lucky enough to be below the impact levels got out.