few ideas

[HueHuey]

i took time to play stick ranger in all ways possible, with all classes and most of weapons, and tried some fancy builds and teams( goldrush + gold medal_⁴ on 4 boxers) and just created this account to spam out all my ideas

1-after some level (55), a card gets 2 or 5% chances to drop from any megaboss, a rebuild card, that resets all stats and gives back your SP
2-afer beating all levels in-game, you can edit any slot in your menu, including equipped weapons, classes, inventory and skill.
That "endgame mode" creates a second savegame file, so when you disable that mode, the "normal mode" savegame is automatically loaded
3-maybe another type of equipment, like armors or a class with a shield and a spear

[Alphaepic]

These ideas are okay but they are mostly unethical.
The first one would ruin the point of there being individual bosses for each card. The rebuild card is actually a kind of good idea, but it would completely undo the point of planning out your SP. The second one would ruin the point of the game and would make everything too easy. You wouldn't even have the fun of testing out new teams.
A new type of equipment like a shield might be nice. It could hinder the attack of the physical weapon but increase your defense.