Because I've played so many MMO's, I always find myself wishing that games had things that it lacks, and I wanted to see what other people find lacking with Mabi.
Heres my list:
Guild System - I find the guild system disappointing (Other than having to pay for it), as I can't set a MOTD (Which is useful for updating members on current events), setup ranks (For better guild organization), have no logs (For seeing who has donated gold, or GP, and who needs to), a guild bank (So members can donate items for other members), guild house (So that members can hangout in a house, rather than at guildstone), guild permissions (So that people other than leader can invite members).
Pet Dungeons - I'd love for a dungeon that is made exclusively for pets. It would probably promote people actually TRAINING their pets (I don't train a lot of my pets, only combat, and ones I need for production skills). It would also give people something to do.
Player Auctions - Yes, Tara has an auction system... But its not the type that would be useful to the players; being able to sell your items via an auction house is generally more helpful, and cheaper than selling from a house/shop.
Gifting! - I hate that you cannot gift on Nexon games now. That means I either need to go out, buy a card, come home, type out the code, and give it to someone, rather than using the money from my paypal (I use a paypal card to buy cards from stores as well though), and just fill out their account name or username.