I would like to present a few ideas for mid-game content.
Player housing, for the cities in TESO, and when a city has reached its population limit, sewers are created in said city. This way players have to rent the plot and harvest the materials. Once a house is placed, the initial price will rise when the player sells it to another player or has stopped paying for his rent. This will give the player who purchases another player house to either rebuild it or keep it the way it is. There is quite a bit of room in the cities.
Player farming. This is so that players can farm and harvest a certain amount of plants and only have a limited amount of livestock so that they can either sell to players or cities of that region. And if they lose some livestock, the can distribute quests through a worker NPC on their farm. I also would like to suggest that things like, cloth plants and herbs for alchemy should be harder to grow.
Music mini game. Players gather together and play a song that is part of the lore (books they must retrieve) and surrounding players get a small buff that stacks up to an hour. e.g, more magicka, health, stamina or a small xp boost, etc.
Creating boats for fishing, or sailing the shores of Tamriel (this might just be a whole other game in itself though). From a small dinghy or rowboat to a small schooner (or larger).
Possible mini game with either dice or cards. Just so that some players can play each other or other NPCs for either gold, information or quests. Intimidate and persuasion could also have more use in this to 'psych out' the NPCs.
Persuasion mini game. A lot of other players probable have brought this up, but it is immersing to have to convince an NPC for information, quests or just to bring their prices down a bit.
Some delves and dungeons would have exits in more than one location before you even finished them in some of the other games. Would it be possible to bring this to TESO?
Horse racing. The ability to set race routes, start and finish lines in a group/party on the map on existing roads. A timer should be included. Also could be used for sail boat racing.This one might be a bad idea, but I thought I'd bring it up anyway.
Traveling NPC merchants should have a small cart, large backpack or something to distinguish them from regular NPCs just walking along the road. If they're going to be bid on, then they should have a higher price, since they're traveling.
Barkeeps to have jobs (similar to quests or dailies) for players.
I also noticed that when it is raining or supposed to be cloudy, that there are still shadows as thought the sun is out. Plus I haven't seen any good fog yet.