NPC Mercs (seen on Everquest) is a good idea for richer yet weaker characters (Make them pets XP), but it would probably be best that the devs work on the main items before implementing Mercenaries.
Seasonal Fruits is another easy-to-program thing, as they already have NPCs going off of a time basis (it's 8PM, time to take a break till 6AM). It could be done via items as well.
Treasure Hunting is done, however it's more of a quest now than anything. Add a Shovel, a few treasure maps, and there you go.
But I will say one thing that is not a good idea. PVP... The reason why I quit WoW? All PVP no PVE. Everyone wanted to do PVP, but no one wanted to do PVE. PVP's fine in moderation, but Blizzard Entertainment took PVP and shoved it up everyone's arse and told you to keep it there. If I truly wanted a game where I was doing PVP all the time, I would play button-mashing fighting games, or Savage (free game, it's all PVP, and a little bit of strategy)