I like that the slimes all had a genetic factor. I didn't read the description of the game and just noticed two slimes were every so slightly different from each other despite being at the same stage.
Viewing post in Professor Lyme's Slime Lab jam comments
Probably the hairstyle. At Level 3 each slime gets a random hairstyle, and, after you fuse two slimes with hairstyle, it's a 50-50 between both for the hairstyle of the fusion.
Or maybe you refer to the perks system. Feeding a fruit to a Level 0 slime grants it a specific Perk (a body part). If you fuse two slimes with the same Perk, the Perk level of the fused slime adds up both values.