It depends on your personal preference. If you want to fully understand the context and backstory of the main game first, it might be better to play the main story before Iki Island. However, if you're eager to explore new content and don't mind some potential spoilers about the main story's themes, you can play Iki Island first.
The Iki Island story in Ghost of Tsushima is about Jin Sakai's journey on Iki Island. It's filled with new challenges, enemies, and mysteries. He has to face a new group of foes and also deal with his own past traumas that are somehow related to the island.
There are likely new enemies in the after story of Iki Island in Ghost of Tsushima. The developers usually introduce tougher foes at this stage to test the skills players have developed throughout the main game. These new enemies could have different attack patterns and weaknesses compared to those in the main story, forcing players to adapt their combat strategies.
If you play Iki Island before the main story, you might be at a bit of a disadvantage in terms of gameplay. You may not have fully grasped all the game mechanics or obtained some of the more advanced skills from the main story. However, if you play it after, you can bring all your mastered skills and upgraded gear, which can make the combat in Iki Island more fluid and easier.
Players can expect more exploration. There might be new areas to discover, hidden secrets, and perhaps some new side quests that add to the overall lore of the game.
In my opinion, it's worth playing. It not only extends the story but also has the potential to introduce new elements to the game. The additional content can keep you engaged if you're a fan of the game's mechanics, story, and the richly detailed world of Tsushima. It can provide a sense of closure or open up new possibilities for the story, depending on how it's crafted.
I wouldn't recommend it. The main story of Ghost of Tsushima is like the backbone of the entire gaming experience. It introduces you to the main characters, their motives, and the overall setting. The DLC usually expands on what has been established in the main story. So, if you play the DLC first, you may miss out on important details and the full emotional impact of the story.