If I was going to set my 90 back up as a display tank rather than a QT I would probably pick a biotope that supported small fish (apex species no more than six inches) and mimic that
Or a single cichlid that maxes out at one foot (green terror, female festae, or such)
-single Oscar
-single Midas or Red devil....although it could outgrow the 90 potentially
-geoophagus with a royal pleco and another midwater schooling fish or Geos paired with green terror
-Flowerhorn
-one of the smaller paeachromis species