This sounds like a good idea. However, what if instead of instantly changing to a different reskin of the map, the map changes realistically? For example, it can have a chance to snow during the winter months (I’m unsure exactly where on the globe TU is supposed to be on) and leaves can fall off of trees during the autumn months. It doesn’t snow on Christmas every year in real life, so a small detail like that can make snow a weather event that people look forward to finally seeing, just like in real life. (Obviously not everybody wants to see snow, but my point is that seasonal weather isn’t a scripted event)
How would this work, then? Since snow may be gathering on the ground, you want it to form piles and sheets on the ground. Why not let players stand still while it’s snowing, just to pop out of said piles? Players should be able to be buried in snow if they let it happen, for example. Getting out of snow should be as simple as jumping out or digging through the snow. Some seasonal weather can form piles and whatnot for players to jump into and in some cases, walk on.
In this case, we have one of the limitations keeping us from having a dynamically-changing lobby out of the way. But what about seasonal decorations in the lobby? Well, a simple solution is to have the decorations be put up by the players themselves! Instead of having it done for them, why not give players the option to decorate the lobby with Christmas lights, etcetera while still preventing players to grief the lobby with custom-made designs (custom-made designs can be made on unranked servers based on server settings)? Decorations can be put up in specific templates that go on each specific part of the map, while only one design can go to a place at a time. Rewards can be given to the players who decorated the lobby to motivate them (think of it as a minigame).
In addition to random event snow, let’s talk about ice. Ice is different as it depends on how cold the internal temperature of the water is. Using a temperature mechanic, it is possible to emulate when and how thick ice may be. Some places in the plaza/condos, such as the pools, could freeze over if the conditions are correct. Perhaps if it is cold enough after it rains we can run on black ice? Keep in mind that all forms of precipitation can be emulated in TU.
As for random events or minigames during specific seasons (and during specific weather types, such as snowball fights), I think they are a good idea as well. Also, non-weather related minigames can happen, too! If you wanted to pile together the leaves scattered on the ground and jump in them, it is possible to do. Building sandcastles may be possible all year long, but minigames or rather, “competitions” are only held in the appropriate season.
In conclusion, seasonal weather should happen realistically just like any other weather type. It shouldn't be a scripted event like how leaves fall off of trees in autumn, but instead it should be a random event, like rain. Decorations can easily be put up by the players in the lobby without the need of another map variation. Seasonal minigames could happen that get players involved in the lobby. Ice can be implemented, just like any other side-effect of weather and season (such as puddles after and during rain, or leaves on the ground in autumn).