I just started playing today and I already got some ideas of improvement.
1) DROPS
Drops are alittle much. Killing 1 bang results in around 15 items which makes it hard to collect the things you actually want (Penya, Questies, 4%/B Cards). Now, this can be fixed if you have a pickup pet to go through it all for you, but even then your inventory gets cluttered with worthless crap that you end up deleting. My solution? Delete things like C and D cards along with 2% and 3% cards. Also, delete Meat Salads and Icecream Cake since Star candy make them obsolete, which brings me to my next idea.
2) STACKING
Some items, pills for example, stack perfectly in your inventory. However, Star candies do not. Because I use both candies and pills, my inventory can become even more unorganized when I have 5 stacks of candies floating around. I'm just using candies as an example, but anything that doesn't already stack up to 9999 should. It all goes back to the inventory being cluttered. I need all the room I can get for more items, so this would be a little thing that would go a long way.
3) BUFFS
I noticed that every buff is given with the buff machine, and at first I thought it was great. However, I started to think about what a nuisance it could become. With all of the buffs that it gives you, you only have 3 open buff slots. Well, every class has at least three self-buffs, so that means that every class would fill all of the open spaces. Red Meteo, Drakul and the other main bosses stun and inflict other damaging moves that take up buff slots, so it's easy to lose your buffs. Get what I'm saying? It could be pretty easily fixed if you 1) Made buff slots unlimited or 2) Took out unimportant buffs like Patience or Protect.
4) QUEST ITEMS
This one isn't a big deal at all, but it's something that I thought of to increase efficiency and make things easier for the players; You could add quest items needed for master and hero quests to shops. Adding Rincruet, Popshift, Dragon Heart, Dragon Teeth, and Mysterious Eyeballs to shop would just make the whole world better. The only problem I can think of with this is people buying mass amounts of quest items then mass hatching pets, but you can slightly increase the price of the questies enough so that to buy a huge amount would be costly.
So yeah, those are my first impression suggestions. Like I said, I only started today so some of these things might already be taken into consideration. I just thought I'd give my two cents.
-Ryan