Easter egg concept:
1. In the lust fight if you don't attempt to hit Lust or damage them once for 20 whole seconds, she taunts you with the kiss and her attacks stop to beckon you closer, if you get close she does the game over animation on you but instead of the game over you receive a tattoo, the tattoo is either A: Usage of UB (maybe 50% chance of getting 2 hearts than 1, and a slight speed boost for 5 seconds), B: transform into lust but weaker.
2. To obtain the alt version of the above, you have to hit Lust when she is doing the taunt when you don't hit her, activating bullet hell mode doubling all projectiles, and once you defeat lust you gain CV (for normal enemies they have to be half health to be used on, for bosses they need to be a quarter or less but can't die from CV, you have to mash to drain the health of the prey inside the balls but you gain faster firing speed and the limit is 2 large prey or 4 normal prey), a better version of UB.
might be hard but somehow I came up with this.