Apotheosis is the largest and most popular content mod in Noita, completely overhauling the game, over doubling the content within it and introducing multiple new mechanics.
This was my first project of such a large scale and it required over 4 years of devotion to get it where it is today.
When I initially started working on Apotheosis it was a challenge communicating with and coordinating a team of developers to help work on the project with me; as we continued to work on it however I developed my leadership skills and now confidently continue to lead the development of Noita's largest mod to over 70,000 subscribers.

Working on Apotheosis has been my biggest learning experience by far, outside of developing my leadership skills I had to learn how to keep such a large project properly organised, how to optimise my code where it mattered so thousands of changes to the game could be loaded in less than a second, how to improve in skill-sets I was weak in and many other things.
I think the most interesting thing to talk about with Apotheosis would be the design process.
When developing apotheosis my #1 concern by far is always how something affects gameplay, my order of priority when adding new content is as follows:

Gameplay is always the top priority by far.
If a new structure is being added to the game and it grabs the players attention it should do so for a reason, otherwise if it doesn't serve a purpose it should sink into the background.
If a magic spell has an audio visual redesign I should ask, is it visually distracting? Does the new audio conflict with any existing sound-cues and mislead the player? Does this make any existing content irrelevent? Does its saturation blend into the background and make it difficult to see?
If a new enemy is added, are its attacks properly indicated? Is its threat level properly indicated? Is the player given the opportunity to escape the enemy if they are intended to have the option to do so? Is the enemy easy to identify from a glance? Does the player feel like they're being treated fairly?
Gameplay, Gameplay, Gameplay is always top priority.
Gameplay trumps everything

It doesn't matter if people are excited for new content if the new content itself turns out to be boring, and you won't get people excited for your next piece of content if the last one was a flop.
Every new addition should integrate smoothly into what already exists, new content should add decisions for the player to make rather than take them away, but also leave the door wide open for future content to integrate into it.
There are countless different situations I could talk about for hours regarding gameplay design but my general philosophy is always thinking about the big picture about how every system, mechanic and piece of content interacts with each other, exponential content is right there waiting to be brought into existence and you only need to make a few systems work with each other to attain it.