In my own opinion, C is the best possible option thus far (we'll see about D). B will be a literal nightmare to deal with on the server. It tracks players who don't sleep, and based on the info we have so far, uses air as spawning spaces. In order to optimize grinders, you need to block all the air blocks at high altitudes, making things like Skyblock swamped with swarms of these things coming at you (they spawn in packs). Imagine playing skyblock, were beds are nearly impossible to get, and then getting attacked by a horde of flying monsters with a large detection range.
A might be interesting, and Minecraft's oceans have always been a bit bland. However, how often do people actually go into an ocean? I don't see it affecting much unless it drops something really useful.
As for C, we have several confirmed features that are quite beneficial to a larger playerbase. They are stationary, but can burrow into blocks (either tunneling for you, or hiding inside blocks), works as a trash can (consumes mobs and items), and even works with enchantments (possibly removing cursed enchantments from your tools/armor?)