In regards to the rules governing when players can enter PvP (with consideration to the announcement of making hostility a mutual agreement), I believe it should be kept simple and without complicated requirements. All non-hardcore characters, assuming they agree to "become hostile" with each other, should be allowed to engage at any place and time.
This system prevents unsolicited "griefing" while maintaining the players' freedom to duke out their differences the old fashioned way.
As for the rules governing an organized and game-sanctioned PvP competition or "arena", I'd like all characters to be able to enter solo or team-based matches based upon player-determined level brackets.
The motivation to compete in PvP should be the enjoyment of the battle itself under "arena" conditions - not prospective titles or loot (though I do though XP might be an appropriate reward).
If that's not the case, I'd like to officially request that any possible item-based rewards perfectly fit the classes to whom they're awarded, and preferably not in the form of an entire suit of armor. Diablo games allow players to customize their own stats as well as skills. Gear does not need to inflate casting classes' HP far beyond what the players have already dedicated to stamina. (Clarification: Please don't design any possible Diablo III PvP rewards in a similar fashion to WoW's arena awards. The radical changes to gameplay that occur when every player is given triple HP completely ruins the class structure, IMO.)
Trading and Itemization The ability to link items through chat is a feature that would definitely bring the Diablo series up to date.
To expand upon that idea, I think a welcomed feature by many would be trading-specific games that allow 20 or so players to interact and barter in-character. Multiple instances of these games could be static and always open to any player - just like a bazaar. Players inside the rooms would benefit from being able to link their items into trade-channels (and perhaps other trade-games if there were a traveling system in place to get from one to the other easily) for larger audiences to browse.
As for what will be traded, I think the player-driven economy should return to a more free-formed structure. This revolves entirely around itemization, so let me explain,
Before D2:LoD, "uniques" were relatively low-leveled, but usually featured "unique" attributes that weren't found on other (even higher level) magical items. During this time, "rares" could be found of every type of item, but could potentially reach higher stats and modifiers than found in the uniques. Rares, if they had the right combination of modifiers, were actually valuable in the economy because of their higher attributes, while uniques were desired because of the truly unique abilities (Example: Iceblink's ability to freeze targets).
After D2:LoD, the amount of uniques more-than-tripled, but they were most desired for their substantially large amounts of +damage, stats, or skills; their unique qualities were no longer the most sought-after aspects. Uniques were simply more-powerful rares, where as rares were now the true unique items with individual lists of modifiers. Runes and the addition of further runewords (1.10) lessened the desire for rares by having unique attributes with even larger amounts of +damage, stats, and/or skills than uniques.
The biggest problem with this design is that the economy is completely driven by high level uniques, runes, and runewords which all have nearly static attributes and are all greater than what you would find on anything else.
I'd love to see a return to a completely free bartering system where any rare a player finds (assuming he or she is a decently high level) has the potential to be genuinely good and/or valuable.
In addition to that, there could be a new tier of magic items (that are equal to greater in difficulty to find than D2's uniques) that contains much higher versions of modifiers that you'd find in rares. These would still be filled with completely random stats and could be one of the strongest weapons in the game to something slightly odd-ball in modifier combinations.
It probably goes without saying that completely preventing "duping" and bots would finally make a balanced economy, regardless of the final itemization.
I think any veteran to D2 will know that storage space can disappear quickly. I've read in interviews with Lead Designer Jay Wilson that the way items are to be carried will be changed so that each item take up 1 slot.
Many people would love to see the limited-space issues addressed, but I think that can be done in a slightly different manner.
I've always thought that the storage chest in town would benefit from allowing items to be reduced to a single slot, reducing the need for many players to create entire characters or accounts simply to hold items for traiding. Furthermore, the storage chest or something similar could be created to allow open access to items from any character on a single account. People will inherently want to give certain items they find to other characters they've made, so why not allow them to do so without having to create private games with the fear of the rooms disappearing between transfers?
From what I've read about the changes to the potion or HP/mana regeneration incentives, I am completely pleased! I'm glad that strategy (beyond just running away when surrounded) will be playing a role for all classes. As a major fan of melee Assassins, I know perfectly well what it's like to rely on my own abilities to regenerate HP/mana (using Cobra Strike).
The reason I bring this up is because potions will no longer need to clutter up inventory space. Players can traverse the mob-filled locations and loot what they think is most important. When their inventories are finally filled, the player is given the ability to chose between multiple smaller items or one larger one.
Potions can be kept only inside the belt.
Gems/runes could have their own bag (or a separate inventory tab - possibly upgradeable).
Charms should have a limited space in their own bag (or a separate inventory tab - possibly upgradeable)
Remaining items can be stored in the traditional inventory. The need to condense every item into a single slot is now gone, while still giving players the feeling of digging through an actual inventory.
Storage space, just as in Resident Evil games, can reduce large bulky items into a single-slot solution inside the stash, while still giving players the satisfactory choice in which items to keep in their inventory. The balance between bulky armors/weapons for trade and multiple small items for vendoring was a great gameplay element.
Mr. Jay Wilson has stated in other interviews that the reason we've not seen any light radius in videos shown thus far is because there would be technical limitations.
Considering that games such as Mythos were able to create fully dynamic shadows and lighting systems, I'm slightly perplexed at this; the hardware requirements for that game aren't particularly demanding.
As a person that's highly fond of the diverse color pallet we've seen so far, I think the inclusion of shadows would actually enhance the vibrancy of the environmental features as well as spell effects. That's not even mentioning that it would likely squelch a large percentage of those who are against the current "art direction".
Adding a light radius that doesn't black-out the edges of the screen (instead, it could simply darken them and hide the enemies) would only increase the atmospheric presence for players. If it's at all possible, I'd love to see it implemented.