Several suggestions.
I have an idea for delves, dungeons and trials. A player goes to a delve with some parchment and maps out the delve by exploring it (Cartography?).This could justify larger more labyrinth like delves. And if players get lost, they can use a teleport gem (which can be purchased in game somewhere) that will port them outside the entrance of the delve that they are in. Players can share their map with a guild or sell it. Could some delves also lead to other dungeons and trials? To add to this, I would like to say that some books scattered around the world drop clues and hints as to where a secret entrance 'might' be to in certain delves to open up a whole new area within said delve. An example would be, a delve in Riften might have a secret entrance, but you have to read a book somewhere in Eastmarch to figure out how to access it. And it could be either you have to start a mini quest chain (please make it difficult) for the area to open up or just buy a already made map from one of the guild vendors.
Could we also get a few zones underground? Like a giant delve like zone, lost city or series of connected caverns. Speaking of zones, do they have to be instanced or can they open world?
I also think NPC vendors should also sell different kinds of potions other than magicka, health and stamina, just to showcase the different kinds of potions that can be made in alchemy. Of course with player made potions being more superior and cheaper.
I would also like to suggest that players can craft any item at any level providing that they have their crafting skill up that high. Like, lets say a level 12 player can create level 43 gear, or a level 23 player can create VR5 potions, providing they can purchase materials from the guild stores. That way some players who just like crafting can craft for a while until they have to venture out to more dangerous areas to get better stats on the gear that they make. Hopefully other players will escort them.
I had mentioned sailing in a previous post. I still think its a good idea, but ZoS, you're the professionals. If you do lean in that direction, I would like to suggest naval battles for both pve and pvp. An archipelago of new zones can add to the naval strategy. These islands can have small port towns for ships and naval quests! The quests types can also include delivery and fishing as well as any general quests. These islands should be uncharted, similar to the unmapped delves mentioned earlier in this post. It might be slightly off the lore map, but I think it would be fun to own a ship and deliver trade goods to other ports or go questing in a ship while exploring the seas of Tamriel. All while defending precious cargo from pirates, raiders, sea monsters and bad weather. Although I would like to see this, I know that this might also be a whole new game in itself. Other online games have done this pretty well, but sadly didn't get a lot of attention for one reason or another.
I noticed that a lot of the forests seem to be lacking in the tree department. I don't know if this is an game engine issue or a design decision, but could ZoS add more trees in 'forested' areas if its possible.
Again, I would like to suggest traveling NPC vendors should have a small cart, large backpack or something to distinguish them from regular NPCs just walking along the road. These NPCs are separate from the guild vendor NPCs.
I had also mentioned player housing and farming. Although I would like to see it, I don't think its a good idea to make it instanced (I mean several players owning the same house via phasing). New zones that have areas that have been 'cleared' via questing can provide more than enough land. Empty space in some of the towns and cities can be used as well. I understand that there would be a finite amount of housing, and that I might not get one, but I can live with that. I would also like to suggest that players can rent rooms of their own in taverns and inns all over Tamriel as a smaller, cheaper alternative to player housing. And let the players decide how much they sell their home for (excluding rented rooms) as a deed in trade chat or a guild store.
I have an idea for neighboring faction zones. I suggest that factions sharing borders have a few areas where factional NPC armies fight periodically and push one another back. This game mechanic could influence NPC prices for buying and selling. In other words, players can join these massive pve battles as dailies or just run in slashing wildly to their own discretion.
Thank you for your consideration in advance.