We can see that this game, like other games, will always be repetitive (Brotato, Vampire Survivors, and others). That said, repetition must be combined with creativity so that players can play and not get bored or bored so quickly. I have a few suggestions:
Item evolution: Obtain the same items or items with similar benefits two or more times so that these items can be combined to create a unique item with the benefit of the previous item + something else. (I remember this mechanic in Vampire Survivors)
Character evolution: Create a store or a category of the store dedicated to improving characters, spending Level to increase base attack, base speed, improve skills, unlock a passive skill, and so on.
Improving the store: As the game progresses, depending on the items, characters, or things you unlock, new things can be purchased in the store.
Create Achievements: A place to view achievements, how to unlock them, and get something in return for unlocking them, such as an item, character, new item to buy in the store, etc.
Soundtrack: Having the option to choose a soundtrack that you like while playing, or each character having their own soundtrack.
Boss: Each boss has a very small chance of dropping a unique item that can only be picked up once per game, and each enemy has their own version of the boss.
Graphic Style: Improving the graphics or making them more unique so that the game has its own visual charm and memorable graphics.
Events: Random events that have a chance to happen every new game that is started, which can also happen when picking up certain items.
These are just suggestions that you will choose which ones to do, redo, take as inspiration, etc. Thank you for your attention!