That's what we do know they are putting into the '24 MM: specifically they used the examples of the Lich and Vampire, creating variants similar to Dragon Age categories, so you can have a weak vampire or wet behind the ears Lich at low Levels.If WotC needs to pad things out more, I would rather they make more variants of existing monsters (more types of goblins, more types of orcs, more types of giants of each type, etc.), similar to @Nixlord's excellent Monster Manual Expanded series, than to give me a bunch of weirdos I won't use.