Well at the moment I feel the NPCs are boring, and speak boring.
I feel we need to liven them up a little
Make them speak differently, more like people.
Buff Pang: "Come to me Noobs, let me make you pro!"
Mayor: "Welcome to the dark side, we have many biscuits for you."
DonationCS: "I give thanks to you for donating, so buy some cheap CS."
CS Seller: "You won't find better items anywhere else, so come buy my stuff!"
Jewelery NPC: "Come let me destroy your items for beautiful jewels!"
PostBox: "Bouncie Bouncie"
Secret Room: "You wish you know what was in here!"
(That's what I have for now, will be posting more soon.
Feel free to post some of your own ideas
)