Actually, I am playing on that size category, which is why I used the term colossus/colossal. This sort of design would be used for colossal creatures (bigger than gargantuan in size, which is currently the largest size creature that the game has used), not for "bosses." This design would not be used to represent any boss creatures. Smaller "boss creatures," such as powerful vampires, liches, NPC wizards, or even younger dragons, would not be "programed" this way. They would be designed with a single stat block. Most of them would use minions of some shape or form in order to make themselves more of a threat. Only the largest of monsters would be designed this way. Or maybe both gargantuan and colossal creatures would be designed this way, but the larger sized creatures would have more parts.
The kind of play that this fosters -- individual opponents being equal match to a team -- is as much ideal for a powerful lich as it is for a dragon, greater size or not. But then, I guess that could be solved by giving the lich multiple turns. I shall droppeth it.
I guess I just don’t agree. I don’t think the lich should be as good at fighting an entire party, alone, as a massive colossus, even if it is an old and powerful lich. No matter how much skill you accrue, enough weaker foes are a very serious threat. That is the bounded accuracy paradigm. A lich should not fight the party alone. A lich that faces a large enough mob should be worried. That is why it should fight along with a group of minions. A mob doesn’t just have to deal with the lich, they also have to deal with its mob of undead minions. The lich along with a group of minions will already play the way an encounter with a colossus will. You figure out what minions and creatures you want to take out first in order to tactically heighten your probability of victory.
The idea is a lot like an RPG I'm lazily homebrewing (naturally, I'm gonna be mining your thoughts for ways to improve my own stuff). So, your thought as to the head (and why people wouldn't attack the head nonstop and ignore all other body parts) is that reducing the head to 0 HP or lower doesn't knock out the whole creature... but arguably, it's still better to pursue the head because if the head dies, the whole beast dies. Chimeras and Hydras aside, of course. What if some colossal creatures (like the dragon) can use other body parts to block attacks aimed at their head, unless the attack has advantage or the dragon is at disadvantage? That way it becomes a matter of "how do we catch the dragon off-guard, so we can plant our swords in its skull?"
Every creature will be designed differently. Something like a dragon, that uses its head in combat, is likely to have a fairly tough head. Some colossal creatures, like a giant, should probably be able to use reactions to soak damage to one body part with another part. The dragon statblock I gave, however, already has a very tough head. Giving it such a power would make it pointless to attack the head. I think a power like that should probably be saved for the theoretical giant design I proposed (where the head is just a weakness with no real powers of its own).
If a wizard launches a fireball at a colossal dragon (and it hit three body parts or more), would each body part get a dexterity save?
Yes, and each would suffer damage based on the result of its save.
What about when a spell like Command is used, that requires a wisdom save; can it target body parts that don't house the thoughts of the creature?
Huh. I would think mind-effecting spells should only affect parts of the creature that house thoughts. The real question is how that mechanic would work with the other parts of the creature. I think my response would be that the entire creature is affected, but on every parts turn the creature can respond to the spell as per normal. So, for example, if a creature normally gets a save every turn then on every parts’ turn the creature should get another save (but the spellcaster casts the spell on the head in order to affect the creature). The designers would have to fiddle with such effects a little until they find a result they like.