4Game

Once you kill Bonemass the bosses two tiers down start getting upscaled to a tougher, deadlier version with new abilties and, possibly, new items.

Kill Bonemass – Eikthyr gets upscaled. "Son of Eikthyr"

Kill Moder – Elder gets upscaled. "Restless Elder"

Kill Yagluth – Bonemass gets upscaled. "Pustulant Bonemass"

Kill The Queen – Moder gets upscaled. "Chillwind Moder"

Just as examples. The upscaled boss would have a different color or be larger or something. It could do to keep the lower-tiers fresh and interesting and this could keep going right up to the last update.

Kill Whatever(Surtur, probably) – Yagluth gets upscaled. "Boneking Yagluth"

Kill Frost Boss – The Queen gets upscaled. "The Last Monarch"

Most biomes will eventually become entirely trivial with player progression. This might give us something exciting to look forward to going back and doing.