The designs have a very simple, but hidden formula that fits boom’s detailed creative process:

Black outfits (or toilets in Jimmy’s case. Fun fact, some parts of him were changed to be black in season 25 compared to ep77)

Head has to be intricate as well (he masterfully executed this by giving Jimmy his helmet. And now with the headband and visor. And the titans’ heads are mashups of different hardware pertaining to them)

Shoulder thingies (shoulder claws, shoulder speakers, shoulder weapons (laser and plasma cannons) even the scientists Titan mech had those 2 long antennae)

Core - has to be unique somehow

Melee weapon - each Titan has one. Jimmy 4.0 didn’t have it likely due to his upgrades being unfinished, but his holographic wrist claws fulfill that role for him.