Let's keep this simple.

Early HM: upgrades to the Eater/Crimera minions. The Eater can upgrade into a Corruptor that vile spits at enemies. The Crimera could upgrade into an Ichor sticker that spews ichor.

EoW minion change- Maybe the segments can vile spit at enemies (or a simpler change is that the minion's spit rate comes from the head and increases per segment.) Right now the BoC one outclasses it a lot.

Skeletron- 3 part summon. Head and 2 hands. Gets stronger with more slots. The head shoots Book of Skulls projectiles while the hands swing at enemies.

Wall of Flesh- a living ball of meat with 2 eyes and a mouth. Pretty simple. Eyes shoot lasers, mouth bites. Not sure if it'd have hungries or leeches tho.

Skeletron Prime-upgrade to skeletron minion but with 4 arms.

Golem (since plantera and EoL already have summons) little golem with rocket fists and laser eyes. Can also squash enemies like a thwomp.

Cultist- maybe a mini cultist that casts miniature versions of the boss' spells?

Moonlord- True Eyes summon that mimics the eyes' attacks or something.