Nice letter 👍
Also nice ideas i would like to see implemented in the game.
Unfortunately, some of them are impossible or too difficult to implement (like seasons and underground).
Seas definitely need more attention. Devs were talking about reworking/upgrading ocean biome. That will probably come as the last update so I believe we need to keep talking about it. I hope new ships will come earlier.
Also mini bosses are great idea and have been suggested a lot of times. I hope devs have this under consideration.
What i believe we need to encourage sailing and exploring are POIs. Something that we need to conquer like towers, bridges, lighthouses, more dungeons with minibosses at the end. Trader could sell maps for these.