I just had a few ideas for the mod that I thought would be cool...
Also it would be fun to see ideas others had as well.

My main idea is that, since the infected Eye of Cthulhu has tentacles coming out of the front, maybe it could have a grab attack where it grabs the player (and maybe rams them into terrain and stuff).

Another is for infected bosses to evolve, and maybe combine as well. For example, if the infected EoC, EoW/BoC, and Skeletron are all in the world when the moon lord is summoned, maybe they could form an "assembled infected moon lord" which would basically be the final boss.

Or maybe some bosses like the Brain of Cthulhu could become stationary when infected, and act as major sources of the spread like supercharged Beckons, and spawn way more monsters than usual.

You could also have some of the smaller mobs latch onto the player and blind them like the nebula brains

Speaking of which, infected pillars would be a really cool thing to see. It would also be a nightmare to fight against. Even more supercharged Beckons than the Brain would be.

Infected Demonite/Crimtane?

Also if you want someone to make art for the mod I am completely willing.