This was the very first thread I found when I looked up the Terraria forums, back as a lurker. And now, having just generated a world where two crimson areas spawned in the middle of the surface jungle, I'm reminded why I support it. I'd love to see the jungle get cross-biomed instead of immunity, but even immunity beats the current state of things. Chlorophyte is useless as a defense mechanism, if you can even call it that, and while I know how to build containment tunnels quickly - the possibility of an evil biome generating in the middle of the jungle is enough for me to turn off autosave and just cheese for a good result. Because once it gets in there, it's a nightmare to cordon off... especially if you're a new hardmode player, because you'll never survive long enough in the underground jungle to dig those hellevators.
Anyway, I see lots of potential in cross-biome enemies here. Illuminants are largely underground, but surface jungle Hallow enemies can be bright and patterned - lots of pretty insect wings and colorful petals. It's a theme that fits the Fair Folk well, so you could throw in a few nature spirits and wayward fae and they'd fit right in. Give the Dryad some less than friendly sisters, perhaps, along with the pitcher plants and pollen-raining giant butterflies. At night, will-o-wisps?
Crimson would be fun. Thriving living flesh. I picture blood-sucking creatures, lampreys and vampire bats and overgrown mosquitos, but also Crimsoned giant ants with bone mandibles, perhaps inflicting Bleeding. (That would probably be a staple, here.) As for the snatchers/man eaters/angry trappers - vines made out of stretching flesh.
Corruption's the one I find hardest to envision, honestly, because it's so antithetical to the jungle, thriving life versus spreading decay - perhaps the reason why the permanent destruction was implemented in the first place. (It makes no sense for Crimson to dry out mud. Crimson... moistens.) Anyway, maybe it could change the jungle theme to a swamp/bog, and populate it accordingly. Giant, thorny slugs. A clinger-like hydra, in the same style as the rest of the corrupted monsters. Some sort of kelpie-like beast? That might be straying a bit far from the core concept, which is diseased versions of jungle enemies. Thornlike protrusions. Eyes where eyes don't belong.
The background plants that normally grow on jungle grass are easy to picture - Hallow would get bright, colorful flora, Crimson would get fleshy abominations, and Corruption would get wilting, brown-green blooms.