1. I really like the "Nesingwary" type quests - where you run into the same NPC as you level. Sometimes, with hilarious consequences and tasks. I think it brings so much joy and immersion to your character and time in game. I would always look forward to seeing the NPC as I progressed through the game.
2. I also like quests with loyalty consequences. For instance, if you help one group who is at odds with their rivals, you are unable to use the rival's camp (NPCs, crafting stations, shops, etc) Makes you really think about which group you will help. (think oracles vs. puppy men)
3. I enjoy quests where you (for example) find pages to a book or parts to a portal - that you can later use to your advantage. Not all players would take the time to collect these difficult items, and you can become known as the one who "has that spell book with the amazing buff" or can "make that portal to the raid entrance" These will really set your character apart because you put in time and effort to complete the tasks required.
4. I hope to see quests where you help an NPC and learn more about their backstory (and maybe how you fit into the story) as time progresses. For instance, you come across an old lady in a dilapidated shack. She is heartbroken because all of her cats have run away. As the game progresses you find and capture the cats - other NPC's can even give you clues and hints regarding the cat locations, if you take the time to speak with them. You have to return to the lady and bring her the captured cats, and will find that she is a powerful mage and she can grant you an exclusive enchantment or emote or title - something distinctive and special.
You can probably tell, I really like long-form and immersive quests. I have so many ideas!
I think there should be a short main quest to follow that takes you through most zones of your faction. And the there should be many optional quests in each zone. So players who like doing quests and exploring the lore can do plenty of quests while players who only want to level up quickly and get to the end-game content can do just that and for example instead of doing 48374837843 quests to level up, they could just grind monsters or run dungeons all the time. I feel like the more choice in how you want to level up the better for the game as more players will be interested in the game and happy with the gameplay.
For those monster grinding fans, I would create something like a Hunting Board. These would be in each city/town. You would choose one of the available monster hunts from it, and you would have to go and kill 50 of that monster. These would reset every 24hours. They would reward gold, exp, and maybe some crafting resources.