A really scary feature would be adding infected creepers, they'd look gross and when they explode they give off particles like how it looks when a splash potion lands. Could you imagine getting chased by one of those things?! It reminds me of the flood from halo
I realy like this symbiosis idea. I would like to have mor of these. Something like the Parasytic Louse from witchery or some kind of creature which replace one organ (maby with baubles/traveller's gear support?). I can imagin myself walking around with a bunch of parasites defending me or increasing my strength :D.
(maby a ender-parasite which teleports you away when attact by an arrow ;D ?)