Based on https://gist.github.com/agorilla/9df052eb1e15d8aea446 , refactored to do prev as well

map-get-next-prev.scss

/// Function to get next map item
/// returns next map item or fallback value if map, key or next item does not exist
///
/// @author Simon Koch, Toby Stokes
///
/// @access public
///
/// @param {Map} $map - Sass list map
/// @param {String} $key - List map key of the current item
/// @param {String} $fallback (false) - Fallback value if map, key or next item does not exist. If this is 'loop' then returns the first item in the list
/// @param {String} $return (value) - Specifies whether to return the key or the value
///
/// @example scss - Usage
/// $map: (
/// 	a: 100px,
/// 	b: 200px
/// 	c: 300px
/// );
///
/// map-get-next($map, a); // returns 200px
/// map-get-next($map, c, loop); // returns 100px
/// map-get-next($map, c, auto); // returns fallback of auto
/// map-get-next($map, b, false, key); // returns c
/// 

@function map-get-next($map, $key, $fallback: false, $return: value) {

  $key-index: get-map-key($map, $key);
  $next-index: $key-index + 1;

  @if $next-index > length($map) {
    @if $fallback = loop {
      $next-index: 1;
    } @else {
      @warn 'No next map item for key #{$key}';
      @return $fallback;
    }
  }

  @return map-match($map, $key, $next-index, $return);

}

@function map-get-prev($map, $key, $fallback: false, $return: value) {

  $key-index: get-map-key($map, $key);
  $next-index: $key-index - 1;

  @if $next-index <= 0 {
    @if $fallback = loop {
      $next-index: length($map);
    } @else {
      @warn 'No next map item for key #{$key}';
      @return $fallback;
    }
  }

  @return map-match($map, $key, $next-index, $return);

}

@function get-map-key($map, $key) {
  // Check if map is valid
  @if type-of($map) == map {
    // Check if key exists in map
    @if map-has-key($map, $key) {
      @return index(map-keys($map), $key);
    }
    @warn 'No valid key #{$key} in map';
    @return $fallback;
  }
  @warn 'No valid map';
  @return $fallback;
}

@function map-match($map, $key, $next-index, $return: value) {
  // Init index counter variable
  $i: 0;
  // Traverse map for key
  @each $map-key, $map-value in $map {
    $i: $i + 1;
    // If next index return next value or key based on $return
    @if $i == $next-index {
      @if $return == key {
        @return $map-key;
      } @else {
        @return $map-value;
      }
    }
  }
}