I had an i idea but you can decide to add it or not.
Mutant dummy
"Aruggss... Nnngh..."
A variant of dummy who accidentally ate uranium rod and then go into the spawner machine. Now they're mutated. Has big health pool (200) but in exchange of low walkspeed. There's a chance of 1 in 30 to spawn a mini version of them with each successful attack. Also in exchange of health loss. Also will delayed the spawnrate more than normal when they spawn.
(Yes i ain't supposed to comment here but this idea MIGHT be a big hit for you)