there aren't very many achievements in this game, and the ones we do have are pretty easy.
Many games have tons of difficult achievements, which people look forward to completing and feeling like they have accomplished stuff on their characters.
There could be some for getting certain amounts of money, some for doing things like boss runs without taking any damage period, some for reaching certain prestige, one for collecting x amount of costumes, one for collecting every 5 star gun in the game, etc etc etc etc.
Also, I think that it would be very fun and motivating if achievements gave rewards. If there is any way they could make that happen, it would likely be done using the in game mail system, sent to the player that met achievement criteria through mail. You could have multiple characters that acquire achievement rewards, simply by meeting the achievement criteria on a different character again. Rewards could include accessories, costumes, etc (mostly non-combat items).
This could also open up many opportunity for them to exercise their pun-making skills, since most achievement names are like that. Judging from things, such as "Punkin king", they would enjoy creating the achievement names.
We need more. +1
Also the rewards should be bound