Ever had a build so wonderful that you'd wish you could play for longer? How about a victory lap?
Simply put, when the player finishes the last level, they could be given an option to continue playing with their current build by putting them back into the first level again. Some random ideas to spice up the victory lap a little:

• Access to the victory lap unlocked after finishing the map 5 times.
  
• Multiply the amount of health enemies have, either by a flat rate of like 200/300/400 percent, or by a calculated amount that would seem fitting in accordance to player's current stats and cards. Could also do the same thing with damage dealt by bots.
  
• Put a price on starting a victory lap by forcing them to choose one of the following: strip half of the owned cards/strip half of the stats-ups/+50% price on all merchant items/give 1 or 2 random cursed cards
  
• Maybe if the player's forced to pick a downside, they could also be given a blessing, either random or by choice? Could be something like: no curses or curses removable at any moment for a price (5k?)/-25% price on all merchant items/random stat raised by 50% OR diskette power doubled/higher chance for spawning golden crates instead of gray ones AND/OR higher chance of finding diskette cases instead of grenade cases in hidden boxes
  
• All-in: if the player chooses to start a victory lap and they die during that run, they'll get half/no XP from both the first run and the victory lap
  
• Assertive bots: instead of bots hiding around waiting for you to get closer/get a visual on you, make them engage you more aggressively.
  
• Give bosses a twist for further challenge (boss spoilers alert). What if Gus Fring/the chicken man had a gun, or if there were TWO of them fighting you at once? What if the tree gave you less time between spike attacks during the first phase, and spawned more enemies during the second one?