I think one of the biggest hurdles in the game is the issues of managing the limited Quest Log space (Currently 25) and managing inventory space (Even for those who have max upgrades and the Crafting Bag for subscribing).
One of the biggest issues I have with the quests are the way that most of your limited space is taken up by quests that essentially always need to be there if you're at-all interested in playing efficiently. I think the following types of quests should be separate and shouldn't be included in this limit:
-Holiday Quests
-Crafting Dailies (x7)
-Battlegrounds Repeatable
-Bounty Repeatable
-Tales of Tribute Repeatable
-Undaunted Pledges (x3)
-Trials/Arena Weeklies
-Daily Guild Quests (Companion) (x3)
-Imperial City Dailies (x7)
-Cyrodiil Main Dailies (x7)
-Dark Brotherhood Contract
Siegemaster Coffers should also have a timer symbol at least indicating that the merit reward is on-cooldown.
If data storage is an issue, I have an alternative idea here. Crafting Dailies, Undaunted Pledges, Trials/Arena Weeklies/Imperial City Dailies which are essentially predetermined or static should be just be synchronous with all players and should just trigger the quest rewards when first-completed for the day, probably with a screen one can bring up showing which ones a player has completed for the day and when each resets. There is a sacrifice here in that players cannot hold onto such quests and do them on another day, but I think this change would keep the game current and would clear up space that's taken up by people hoarding stale daily quests for extended periods of time.
Also, for quests, updating the quest log and adding a minimap with quest indicators could be useful as well. Adding a quest log section showing completed quests. Adding anything to signify that a quest is in progress rather than just started would be nice, like perhaps a color update once a quest's description has been updated beyond what it is when it's first started. This would allow players to confidently clear their quest log without worrying that they're clearing quests they have made significant progress on.
To further optimize questing just a little bit, it would be nice to tweak companions and allow an easy way to see the following:
-Companion Guild levels
-Companion special passive
-Companion rapport
-Companion questline status (Completed/incomplete)
Currently, for inventory, the options are purchasing and utilizing the coffers, and even these are a bit needlessly tedious requiring two loading screens to get into and out of your house. After this, you're left with a couple of awkward workarounds, namely creating mule characters that hold various junk items. You could also create a private guild to store 500 items, though this requires you to get 9 other players to join and stay who are willing to waste one of their 5 guild slots. This works, but I don't think these should be an option that players should be pushed towards.
It would also be nice if we could rework various items to make the inventory a bit easier to manage.
Key Fragments -> Currency
Vault Keys -> Currency
Siege of Cyrodiil Merit -> Currency
Runebox Fragments -> Create a new section [Collectibles Fragments] for these sorts of items
Lockpick -> Create a new section [Tools] for these sorts of items
Repair Kits -> Create a new section [Tools] for these sorts of items
Also, it would be nice if disguises could be reworked. Upon completion of a quest you should unlock the disguise as a wearable costume.
This might also be a stretch, but I think Furniture should also be weightless given how it functions in the game. Currently I have to use houses as junkyards for furniture and I don't think this is an ideal solution. Alternatively, I think you could make a furnishing bank that increases in size as you acquire additional housing in the game. Placing junk in your houses is a sloppy workaround at the moment.
I think the following items should also be weightless:
-Dyes
-Scrolls
-Surveys
-Motifs
-Style Pages
-Items left over after completing quests
-Disguises (Quest Items)
-Museum Pieces (Quest Items)
Also the following crafting materials:
-Durables - Quality Improvement Materials
-Durables - Style Materials
-Durables - Trait Materials
-Alchemy - Solvents
-Enchanting - Aspects
-Provisioning - Bait
-Furnishing Parts
One-of-a-kind items should also be reconstructible after being destroyed.
I think also that your storage coffers should be accessible from the inventory screen as well. Maybe this changes their functionality too much, but I think all that is cut out is the fast travel to the house to get to the chests. Perhaps you should be able to place items into these from anywhere, but you must travel to a house to retrieve said items, giving some sort of unique inventory management strategy to using these while cutting the tedium in half.
"Unlimited" item stacks should be increased in stack size as much as would be reasonable without sacrificing game stability (Currently ~4.2 billion)
I think the following items should have their stack size increased
-Coffers (1 -> 10)
-Treasure Map (1 -> 5)
-Junk Items (100 -> Unlimited)
-Crafting Items (200 -> 1200)
-Cyrodiil Repair Kits (100 -> Unlimited)
-Soul Gem (200 -> 1000)
-Poisons (1000 -> Unlimited)
-Potions (Various -> 200)
Additional Quality of Life features that should be in vanilla:
-Quick-sell junk (With options to toggle Ornate items, Treasures, "Sell to a merchant" items, Monster Trophies, Traitless Gear, Potions based on rarity, Food based on rarity)
-Quick-deconstruct equipment (With options to toggle Intricate Gear, Gear based on rarity, Traitless Fear
-Auto-deposit (With options to toggle Treasure Maps, Researchable Gear, Companion Gear, Furnishings, Writs, Disguises)
For those with many characters, it would be very handy if the game could determine if a gear's trait is researchable for any of your characters, or if it is known by all.
Siegemaster's Coffers also function as Character Bound even though they are meant to be Bound. They should be tradeable to other characters on the account.
I also think surveys should be reworked in general. There should be a way to spawn in the resource nodes without moving/reloading.
Guild Store listings should be increased maybe to 100. 30 feels too limiting given there's already taxes on each listing and guild traders are bidded upon.
Allow writs to be abandoned without destroying the writ, and show what's in a writ before you accept it.