Add xl step to bootstrap sass. Should be after bootstrap.scss.

_bootstrap-sass-xl.scss

/* Place this after bootstrap.scss */

$screen-xl:                  1560px !default;
$screen-xl-min:              $screen-xl !default;
$screen-xl-desktop:          $screen-xl-min !default;
$screen-lg-max:              ($screen-xl-min - 1) !default;
$container-xlarge-desktop:   (1530px + $grid-gutter-width) !default;
$container-xl:               $container-xlarge-desktop !default;

.container {
  // @include container-fixed; No need for, already done.
  @media (min-width: $screen-xl-min) {
    width: $container-xl;
  }
}

// xLarge grid
//
// Columns, offsets, pushes, and pulls for the large desktop device range.

@media (min-width: $screen-xl-min) {
  @include make-grid(xl);
}

// Generate the xlarge columns
@mixin make-xl-column($columns, $gutter: $grid-gutter-width) {
  position: relative;
  min-height: 1px;
  padding-left:  ($gutter / 2);
  padding-right: ($gutter / 2);

  @media (min-width: $screen-xl-min) {
    float: left;
    width: percentage(($columns / $grid-columns));
  }
}
@mixin make-xl-column-offset($columns) {
  @media (min-width: $screen-xl-min) {
    margin-left: percentage(($columns / $grid-columns));
  }
}
@mixin make-xl-column-push($columns) {
  @media (min-width: $screen-xl-min) {
    left: percentage(($columns / $grid-columns));
  }
}
@mixin make-xl-column-pull($columns) {
  @media (min-width: $screen-xl-min) {
    right: percentage(($columns / $grid-columns));
  }
}

@mixin make-grid-columns($i: 1, $list: ".col-xl-#{$i}") {
  @for $i from (1 + 1) through $grid-columns {
    $list: "#{$list}, .col-xl-#{$i}";
  }
  #{$list} {
    position: relative;
    // Prevent columns from collapsing when empty
    min-height: 1px;
    // Inner gutter via padding
    padding-left:  ($grid-gutter-width / 2);
    padding-right: ($grid-gutter-width / 2);
  }
}

@include make-grid-columns;

@include responsive-invisibility('.visible-xl');

.visible-xl-block,
.visible-xl-inline,
.visible-xl-inline-block {
  display: none !important;
}

@media (min-width: $screen-xl-min) {
  @include responsive-invisibility('.visible-lg');
  @include responsive-visibility('.visible-xl');
}
.visible-xl-block {
  @media (min-width: $screen-xl-min) {
    display: block !important;
  }
}
.visible-xl-inline {
  @media (min-width: $screen-xl-min) {
    display: inline !important;
  }
}
.visible-xl-inline-block {
  @media (min-width: $screen-xl-min) {
    display: inline-block !important;
  }
}

@media (min-width: $screen-lg-min) and (max-width: $screen-lg-max) {
  @include responsive-invisibility('.hidden-lg');
}

@media (min-width: $screen-xl-min) {
  @include responsive-invisibility('.hidden-xl');
  @include responsive-visibility('.hidden-lg');

  .visible-lg-block,
  .visible-lg-inline,
  .visible-lg-inline-block {
    display: none !important;
  }
}

Nice work, are you thinking of creating a bootstrap 4 version with the different mixins and screen vars?

@bruceconlon I haven't used version 4 yet. Might make a version for it whenever it comes in need for a project.