Goliath goes to godliath because he kills enemies, let him do scaling damage as he goes berserk, making him able to kill enemies, AND become a more challenging foe
also he gets faster as he stacks higher
Vermivorous, make us inject eridium in each evolution pod, and you pay a total of idk 50 ? eridium to make him happen also need to keep the varkid alive while damaging it to a certain point, you could propably buff these eridium enhanced varkids so it s harder to do.
A robot who stacks by taking the parts of fallen other robots, building himself up and eventually becoming this bigass super bot
A ratch who eats badass ratches ( when they are on the ground, ratches eat corpses already, its inda a wasted oppertunity
skagzilla, an alpha skag eats eridium either piles you can mine, or of fallen other skags, also loot such as guns granade mods, shields.
this can also be done good with the gun gun.
mothrak, you charge a badass prismatic rakk or whatever they are called with all elements. a little like the cube on necro, depending on what immunity they have at the moment, it requires a few elements on hand, and you need to shoot it as it is immune to it.
one of necros wildelife, you could incorporate one of the eridien artifacts to do this, lets say you melee one of them to peel away its shell or whatever, and as you break the eridium shell it grows perhaps linked to some sort of elemental puzzle