Indoor malls defined American retail for much of the last century. Now, bills are coming due nationwide for billions of dollars in debt tied to malls. What will these properties look like in the future, across the U.S. and in Austin? Some retail experts suggest augmenting existing malls by, for example, turning huge parking lots into apartment complexes. Others envision completely remaking indoor malls, like what can be found at Highland in North Austin. At stake are hundreds of acres, thousands of jobs and millions of dollars in property taxes for local governments.