PvP - I always thought the ear idea for killing players in pvp was great. I believe that they should bring in a more in depth trophy system for pvp. Maybe keeping track of who you have killed up to a certain amount of people of your choice. Like keeping a top 5, 10, 20, or maybe not have a limit. Being a lover of pvp content, I myself would love to have a trophy tab somewhere to see the people I have killed, or memorable people I have killed, or even an amount of times I have killed or have been killed by people. They could really go in depth with this also as to classes, levels, and such that you have killed the most also. An idea to go along with that maybe would be able to gain a pvp bonus for having a trophy from a player up to a certain amount, like 1% more damage for having more kills on that player than they have on you. The possibilites of that would be vast also.
Trading - Trading was always a pain to me in D2, through all parts. The trade channel was a nice implent, before it died. I think an auction house type trade system would take some fun out of the game. I think that a bazaar type system would go better. A way that you could broadcast to players in your games what you have for sale, and maybe being able to add notes as to what you would like to trade them for, or a list of items you are wanting to buy.
*note* Sorry if any of these ideas have been taken already, replied first, reading posts after I hit Submit hehe