Warning!: TL/DR - I broke it into 3 sections for ease of reading/interest, Enjoy!
~~GENERAL IMPROVEMENTS:~~
Cross-city chat channel:
- Eliminates the need to be in 1 main city to see for shouts/discussions.
Auction House:
- Cross-city AH, eliminates having to sell items in less frequented market locations thus increasing more supply in 1 location and price competition.
- Remove being "kicked out" AH when purchasing items (as is the case in current ver.)
More optional chat tabs:
- Allows more options (i.e General/Whisper/LS, etc.).
Notifications:
- Have sold/unsold items to be collected. (Special AH icon)
- Friends list members/LS members go online/offline.
- Mail from other players (v2.0). (Special mail icon)
Linkables:
- Ability to link to chat: Achievements, items, craft recipes, etc.
Note: Currently unless you look up an item, when it drops you have no idea what stats are unless familiar with item. Being able to click on it and see the icon + stats = great! Also allows to show off noteworthy achievements!
Fun Achievements!
- Self explanatory.
Give World NMs noteworthy drops
- Ex: Special items for craft, equipment, etc.
NPC Driven Events
- Hamlet is good but like any event in-game currently requires 8 players & gives solo players/players unable to get full parties together, not many options to do in "downtime." Beseiged (or similar) was a nice touch for FFXI & if provide currency/points for rewards would provide players with incentive & something to do whether 1-8 players.
~~MECHANICS IMPROVEMENT:~~
"Unable to cast while moving" ~ AFTER having just stopped moving...
- Should be able to cast the second you stop using directional movement on keyboard/mouse/controller.
Crowd Control aka "CC"
- Other than Weapon skill Stun/Sleep.
- CC break ability/item for players (with a short Cooldown).
Instant Skills
- Currently aren't so "instant," using self buff/heal skills trigger an animation motion that prevents auto-attack/movement.
Tank multi/single target tanking
- Skills for single target threat control (which is decent at the moment)
- Skills for multi target threat control (needs MAJOR improvement)
Note: giving tanks "AOE" moves to tank reduces tanking to spamming aoe without regard for positioning. Conal/cleave skills would be much better requiring tanks to position & makes better use of skills such as "Cover" & "Collusion" as party placement will be less chaotic.
Target of target Option + Focus Target
- When targeting another player able to see what other player has targeted.
(PvE) Helps others assist player on their target.
(PvP) Huge contributor to knowing if you are being selected as a target.
Focus target:
(PvE) being able to have boss as "focus" to determine HP, while being able to target party members is beneficial especially for bosses that may have moves at certain %.
(PvP) Many uses!
~~PERSONAL SUGGESTIONS:~~
1. More Low level gear options: 1~25 gear options are extremly limited whether Npc/AH.
2. More endgame gear to choose from: Darklight is pretty much it for many classes. WHM gear is extremely limited to a few pieces of items.
3. Better looking gear design: Melee w/ hauberg, felt trousers, explorer's moc, melded hands = looks so mixed & matched with cloth-like armor mixed with heavy-like armor. Basically allow plate/mail wearing classes to have gear that looks like plate/mail. Cloth wearers to wear cloth-like gear, etc.! Basically if you're not wearing Darklight/AF/Crafting set gear you look retarded 
4. More definitive dps/tank stances for Warrior: allowing a well-geared warrior to compete with a well geared DD. No, Im not asking for a tank that can do a DD's job, but for warrior to have 2 "branches" of play DPS (DD) or Tank. Currently Berserk (dps stance) & Rampart (tank stance) does not provide much contrast in playstyle & MOST if not all wars fall into tank role. Berserk increase falls off if take damage (AOE FTL??) perhaps a shield version for warrior for tanking would be nice? Idk.
5. Don't simply raise Max level or Drastically reduce drop rates: Make more strategic game play, not more grinding due to low drops in effort to keep people "interested" in playing.