It seems that a few people (me included) have asked about some modding API on Discord. After very short talk with Cairn4 I started working on some modding API. It's nowhere near complete but I am trying to draft some idea of how it should work. Note that I come from Minecraft background so I am heavily influenced by how it works :P
Currently I have managed to get the original game loaded in IDEA with patched in calls to a custom "ModLoader" code that does barely nothing and started work on some Event Bus which should allow interaction with the base game (events like game ticks, items, blocks, ... - mostly in TODO). I would love other's input on what they want to do with modding API so I know where to focus initially. It will load Java JARs from mods/ folder.
Currently it's hosted in BitBucket (not GitHub because originally private projects) on these two projects:
https://bitbucket.org/CallMeFoxie/mewnbasemodded/src/master/ (code with idea how modloader should work and base game patches and one or two working events)
https://bitbucket.org/CallMeFoxie/mewnbasemod/src/master/ (sample mod that just prints something at the beginning of each game tick)
So far it cannot do much but - opinions? Is EventBus wrong and do you want something else? (I prefer it because later on we can take care of loading order and dependencies.)
Note that I am not a Java programmer by day, not even by night, it's just something I threw together over a few hours.