It would be a really cool idea if there were Cosmetics that you could achieve at Wave 300 of Wave Defence on Nightmare difficulty. This will in turn make Wave Defence a popular gamemode again. I have thought of a couple of Cosmetic idea names. I don't really have a design idea for most of them, though. That will be up to your imagination:

Corrupted Sand Peasant; it looks like the Spiritual Mage, but jet black, with purple crystals and eyes instead of white ones.

Knight of Ice; up to your imagination.

Seafaring Swashbuckler; up to your imagination.

Wise Wizard; he looks like removed enchantment wizard.

Underworld Gatekeeper; up to your imagination.

Martial Arts Master; up to your imagination.

Canals Marksman; up to your imagination.

THERE IS CURRENTLY NO WAVE DEFENCE FOR THE AREAS BELOW:

Sea Serpent Hunter; up to your imagination.

Robot Engineer; up to your imagination.

(As time goes on, I might edit this post to change some parts up.)