powerofthree's Seasons of Skyrim is an amazing mod, but in one respect I found it a bit lacking: By default, it does not include all exterior worldspaces of Skyrim. For some of them, it was probably an oversight, whereas for others it was likely deemed unnecessary. In any case, I find it immersion breaking to not include all, so here is a solution (for those who worry about disk space, the overhead from the extra worldspaces is quite small). The only worldspace I excluded was the Ancestor's Glade, where I don't think it would have pulled off well (and it is quite secluded after all).

Anyway, adding more worldspaces are just some simple edits to the ini files, but in my opinion the seasonal experience is not complete if the snow does not melt in most places in summer. Mefariah provided exactly that with their Seasonal Landscapes - Unfrozen mod, but to extend that to more worldspaces requires a lot more work. Said and done - I have done exactly that. While at it, I could not resist noticing that the originally snowy areas are a bit monotonic in summer, so I added a few elements to enhance that experience (optionally to this mod). In particular, Mefariah had replaced all glaciers with rock in summer to make them blend better with the terrain. They included an option to keep the ice in summer, but that looks bad against the grass. Realism is important to me, so I modified the terrain around the glaciers to make it blend with the ice instead, even though that required quite some work.

On top of that, I realised that the snow atop the Throat of the World is not likely to melt in summer (it is the tallest mountain in Tamriel after all, according to Skyrim lore), so in addition I have added an option to preserve the snow at the peak year round.

As work has proceeded, I have also expanded a bit on other features from Seasons of Skyrim, which are not fully related to the addition of extra worldspaces, but I think still too minor to deserve their own mod page. In particular, I have fixed a couple of things in Seasonal Landscapes and started integrated it with a few other mods.

Features included in this mod, most of them optional:

Worldspace additions:

• Seasons extended to Dragonsreach, Bloated Man's Grotto, Skuldafn, Labyrinthian Maze, Giant's Grove, and Castle Volkihar Courtyard.
• For Unfrozen, this is further extended to Windhelm Pit, Japhet's Folly, and Southfringe Sanctum.
• Additional option to include Forgotten Vale. For regular Seasons, this adds seasonal changes to the enchanted forest, but for the full experience, I recommend to include this with Unfrozen.
• Unfrozen option to the custom worldspace used for the bear hunt quest included with Capital Windhelm Expansion.
• Support for the extra worldspaces added by Immersive College of Winterhold and Immersive Fort Dawnguard.
  

Other landscape features:

• Ice-like glaciers in summer and necessary terrain edits. Comes with custom meshes (where necessary) based on Vanilla, Fixed Mesh Lighting, or Icy Mesh Remaster.
• Throat of the World snow cap.
• Icy waterways in Bloated Man's Grotto and Castle Volkihar Courtyard.
• Seasonal transforms of winter aspens during Spring and Autumn for Unfrozen (mostly affects Forgotten Vale). Requires Seasonal Aspen Trees.
  

Other features:

• Minor additions to Seasonal Landscapes to blend with Forgotten Vale.
• Custom form swaps for some environmental objects during the winter season to blend better with the landscape changes. Also included swaps for Civil War objects and Hearthfire clay pools, which were completely missing from the Seasons plugin.
• Patches for Majestic Mountains for the new objects, with an option for Simplicity of Snow compatibility (without Majestic Mountains, Simplicity of Snow is compatible out of the box).
• Seasonal Alchemy and Seasonal Landscapes extended to Hearthfire plants (only includes plants not covered by Seasonal Alchemy Add-on - Critters and Fungi and Farm Crops). This will not affect plants you plant yourself, but is still useful since many mods add plants from Hearthfires.
• A consistency patch between Seasonal Landscapes and Seasonal Alchemy, making the latter use seasonal textures where relevant. Choose between Vanilla assets and assets based on Mari's flora (the latter includes texture replacers for the original Seasonal Alchemy mod). Optional ENB Light meshes are included as well.
• LOD fixes for some objects from Seasonal Landscapes and Seasonal Landscapes - Unfrozen.
• Fixes for two of the Riften land meshes provided for spring/summer by Seasonal Landscapes, adding a dirt texture to the (obviously) well-used
  paths. Addition of an autumn form swap for the Solitude bridge.
• Blacklists for some objects that should never have snow, in particular embers and rotating objects.
  

Also includes a resource file for your Unfrozen modding needs. Includes form swaps for (more or less) all snowy models not included in Seasonal Landscapes - Unfrozen, that are placed in any of the worldspaces included here or with the original mod, as well as some worldspaces from other mods.

Recommendations:
Apart from the mods listed above, I recommend using Seasonal Weathers Framework - Seasons support for Weathers Mods. You will have to do a bit of patching yourself, especially if using several mods that affect region entries, but it is totally worth it. The provided patches at the mod page are a good starting point, but depending on your weather mod, you may want to add additional weather types to some region (in particular if you want rain in summer in snowy regions). I also recommend expanding the framework to the four Forgotten Vale regions, DLC01WeatherCanyon, DLC01WeatherGrove, DLC01WeatherIce, andDLC01WeatherPlayground, since those are generally not covered by the patches.

Compatibility:
The mods included here have surprisingly few conflicts with other mods, given how many cells they edit. Patches for JK's Skyrim, Helgen Reborn, Winterhold Quest Expansion, Immersive Fort Dawnguard, and COTN Winterhold + JK's Skyrim are available here: Seasonal Landscapes - Unfrozen - More Patches. I don't think I will make any patch for only COTN Winterhold, but we will see.

No other mods in my quite large mod list required a patch, but I am not saying you will not need patches for mods I have not checked out (SSEEdit is your best friend here - I cannot tell beforehand whether these mods are compatible with mod x). However, it is not likely that anything other than the glacier option and possibly the snow cap will conflict with many mods.

DynDOLOD notice (updated 25 February 2024):
For the best experience, you will have to run both xLODGen and DynDOLOD on all worldspaces included here that have their own lod (that is Skuldafn, Japhet's Folly, Forgotten Vale, and the Windhelm Bear Quest worldspace), in addition to the ones already included with Seasons and Unfrozen. This will require quite a lot of disk space (I ended up at 20 GB with high quality meshes and textures), but this is mostly from Tamriel and Solstheim, so you will not be able to escape it anyway. However, it should be possible to save disk space by packing the files into bsa archives.

DynDOLOD can be quite challenging to use even for an experienced modder, but I am not an expert, so don't turn to me for help if you run into issues. I myself previously had problems with LOD models not unloading at close distance during summer. This is apparently a common issue, and is due to a bug in an older version of Seasons of Skyrim. The issue has been fixed in version 1.8.2, so make sure to update if experiencing this.

Regarding bug reports:
If you find any issues, don't hesitate to report, with one exception. I am well aware that there still remains bleeding grass at places in the ice mod. If I have missed something that looks horrible, feel free to report it, but I don't want any reports for minor bleeding. If you want to fix it yourself, feel free to.

What next? (updated 2 November 2023)
Currently, I am working on a combined blacklist/mesh fix mod to get rid of poorly placed snow in winter both for this mod and others, which I plan to release as a separate mod. For this mod, I plan to heavily expand on the list of environmental form swaps, since I have noticed that the main form swap from Seasons of Skyrim generally does a quite poor job with landscape assets. It is not on top of my priority list though, so it might take a while until I get to it.

I am also working on patching a couple of bigger mods with Seasons, which I think deserve their own pages, starting with VIGILANT and Legacy of the Dragonborn. The Vigilant mod is finished, and can be found here: Seasons for Vigilant. For the Legacy of the Dragonborn, a first version is now released, and can be found here: Seasons for Legacy of the Dragonborn. This will receive more features in future.

Acknowledgement:
I would like to thank powerofthree and Mefariah for their work on the mods this work derive on. Without it, this would not have been possible.