I had an interesting idea for trading that I think Blizzard would be interested in. I remember hearing Jay Wilson say how they'd like to find a way to trade between players without having to be in the same game, or having to go out and find a game that is selling what you want.
Diablo III should have Marketplace game types on battle.net.
Marketplace games would be limited and hosted by the Battle.net servers. The games would be open 24 hours a day, and only 10 players can join per game, but the Marketplace is a predetermined map, and you cannot explore out of the city (Caldeum). Once the player is in the game, they can either put one of their items on the Marketplace and have it be open to view on all the Marketplace games, or they can browse the categorized market carts and shops in town. Determining on the rarity and strength of items for sale, they will be sold in different locations in town.
For example, a Stone of Jordan will be sold in the King's Royal Exchange. And an inferior sash will be sold on a poor man's market cart.
NOTE: An item in the Marketplace is only visible if the player that owns that item is online.
How would you bid for an item or try to buy it?
Well, that's kind of hard to explain, but I think I can make you understand. Upon viewing an item of interest, you can view it's stats, the owner, and even preview it on your character (gotta make sure it looks good!), and on the interface, there will be a button that will be named "Bid/buy". Upon clicking the button, it brings up another "Offering" interface that allows you to offer items or gold for the item. After offering items or gold for the item, the player can click the "Bid" button on the interface and that will send a private message to the owner of the item (who is currently online), and they can view your bid in-game, or on the Battle.net chat screen. If they approve the bid, they can authorize it with clicking an "Accept Bid" button on their "View Bid" interface.
Once a trade is made, the player in the Marketplace game automatically gets the item, and the player that sold the item (who is probably not in the Marketplace game) will need to go to their stash to recieve the other players bid.
It all sounds complicated, but I assure you it's much easier than it sounds - even easier than WoW's auction house setup. And if you don't like trading with the Marketplace games, you can always do it Diablo II style in a seperate, player-made game.
The only bad part about the Marketplace is you need to be online for people to view your items for sale. However, that will keep players on Battle.net longer, therefore increasing play time. Would Blizzard want players to stay on their Battle.net servers? I think they would...
If the developers could find a way to implement "PvP-only" based games, they could limit the area of the map much smaller, and place more than 4-5 players in a game. PvP-only games would hold 8-10 players instead - therefore making teams larger and more advanced to play against.
Here are a just a few of my many ideas for PvP:
Hostile Free-for-all (Respawn after Death)
All players in the game are hostile against each other and will fight for the death. Hostile Free-for-all will be held in one of a number of different Coliseums, as well as open land, and dungeons, where players battle in a fixed (or randomized) area. Lut Gholein's Arena for example could be an Arena map. The Bloor Moor could be an outdoor randomized map - as well as a classic look back at Diablo II's most popular killing fields! Time limits and score can be kept for "Ranked Matches" (That's where the competitive PvP takes place).
Hostile Team Free-for-all (Respawn after Death)
Teams of 2, 3, 4, or 5 are hostile against each other and fight for the death. Team Free-for-all would take place in small outdoor, dungeon, or even arena maps - just like regular Free-for-all. Time limits and score can be kept for "Ranked Matches".
Hostile Capture the Flag (Or perhaps, Capture the Gem or something of the like)(Respawn after Death)
Teams of 2, 3, 4, or 5 are hostile against each other and must fight to capture the opponents flag and bring it back to their shrine. Upon retrieval of the flag, the winning team scores 1 point, and recieves a small buff to attributes, armor, attack rating, or even skill levels. Each buff is different and random as the score goes higher. Since the gameplay will be intensly fast and relatively easy, buffs will stack later on in the game and grant the players stronger abilities. If Capture the Flag is designed and plays out like I am thinking, you could seem games with scores of 100 or more in a one-hour game. Time limits and score can be kept for "Ranked Matches". Note - successfully capturing a flag requires your own flag to still be in it's shrine. Attacking while carrying the flag is legal. Multiple flags can be carried by one person. Team Capture the Flag takes place in an arena, dungeon, or outdoor map.
Free-for-all Capture the Flag (Respawn after Death)
All players are hostile against each other and must fight to capture any of the opponents flags and bring it back to their shrine. Upon retrieval of the flag, the scoring player scores 1 point, and recieves a small buff. Each buff is different and random as the score goes higher. Since the gameplay will be even faster than Team Capture the Flag, buffs will stack later on in the game and grant the players super-strong abilities much like Team Capture-the-flag. Time limits and score can be kept for "Ranked Matches". Note - successfully capturing a flag requires your own flag to still be in it's shrine. Attacking while carrying the flag is legal. Multiple flags can be carried by one person. Free-for-all Capture the Flag takes place in an arena, dungeon, or outdoor map.
Elimination (Respawn after round ends)
All players are hostile against each other and must fight to be the last one standing. Time limits and score can be kept for "Ranked Matches". Elimination takes place in an arena, dungeon, or outdoor map.
Team Elimination (Team respawns after round ends)
Same as Elimination, only with teams of 2, 3, 4, or 5 players.