Today’s daily fractals can be obtained from https://api.guildwars2.com/v2/achievements/categories/88
Sample output:
{
  "id": 88,
  "name": "Daily Fractals",
  "description": "",
  "order": 2,
  "icon": "https://render.guildwars2.com/file/4A5834E40CDC6A0C44085B1F697565002D71CD47/1228226.png",
  "achievements": [
    2229,
    2591,
    2929,
    2889,
    2950,
    2947,
    2954,
    2971,
    2986,
    2923,
    2896,
    2907,
    2897,
    2941,
    3203
  ]
}
Since fractals are not part of the v2/achievements/daily structure, tomorrow’s dailies cannot be obtained from v2/achievements/daily/tomorrow.
I propose that the v2/achievements/daily object includes an additional “fractal” property containing simply the “achievements” array, and likewise for v2/achievements/daily/tomorrow
Example:
{
  "pve": [ ... ], // Keep these as is
  "pvp": [ ... ],
  "wvw": [ ... ],
  "special": [ ... ],
  "fractal": [ // Just the array of IDs, no level nor required_access
    2229,
    2591,
    2929,
    2889,
    2950,
    2947,
    2954,
    2971,
    2986,
    2923,
    2896,
    2907,
    2897,
    2941,
    3203
  ]
}
Although the fractals repetitively rotate and can be manually hardcoded to predict tomorrow’s dailies, having an automated API saves developers and players from having to wait 2 weeks every so often Anet changes the fractal rotations.