A shocker hulk. Big light radius, every few turns or so it’ll discharge an electrical field around them, which can damage and increase its light radius for a single turn. When it dies, it also creates a big electrical discharge. Very dangerous if player has no protection against electricity. Also, it’s a hulk so it can smush you to death.
Zombie behemoth. Next step after hulk, perhaps? Loses speed but becomes as tough as a shoggoth and hits even harder than a hulk. Big enough that it’s steps generate a lot of noise. Maybe a grab attack where it tries to squeeze you to death.
Some sort of evolution to the boomer. Explodes into a very sticky mass. Aside from the loud noise it generates getting caught in the goo makes it harder to breathe. It has a spit shot that can temporarily increase mouth encumbrance, and it’s death boom can possibly cut off air for a while, akin to losing oxygen when you swim too long.
A long crawler. A crawler zombie who has developed long and powerful arms to cope with the loss of leg.
Fanged zombie. Zombie which has developed a very strong jaw. Only as tough as a tough zombie maybe, but has a very sharp bite attack, can tear through armor.
Maybe a diseased zombie. Like a step up from decayed. Explodes in a cloud of gas, which could create some sort of debilitating disease for a long while. If it can do a bite infection, instead of blue it already reaches the green stage of infection.
Speeder zombie. Fast attacks, pretty much it. Attack is like the quick attacks of krav maga, and it can hit you multiple times in one turn if you’re badly slowed down.
A playing dead zombie. Looks like a corpse, but if you go too near it leaps up and does a powerful attack… like, I dunno, it like lands on you or something. Only way to avoid the attack is to hit it from a distance to get them to wake up and attack you like a normal zombie.
Regenerator zombie. Has the regenerative properties of the shoggoth, with the added trait of generating blobs upon hit.