Does anyone know of a mod or anything that will give me old cave generation - with more caves and trees growing underground, etc? Or anything to make caves a bit more interesting - cave biomes would be interesting and I feel like maybe I've heard of that before?
I also have a mod, TMCW, that actually adds new types of caves but it is only for 1.6.4, largely because I don't feel the need to update to newer versions (plus, the mod adds some features present in newer versions, even a form of attack cooldown which was designed to not interfere with the ability to attack multiple mobs in quick succession. My opinion on updating mods is if they add enough of their own content they can be seen as a separate version of the game with its own updates).
I have not updated the mods in the link given above since 1.9 or earlier, including a mod based on TMCW (which also does not include much of the stuff I've added underground since the first version) and I'm not sure if I still can since they updated Minecraft to Java 8 and even before I could not make certain changes with Java Bytecode Editor as it was written for Java 5 (it would mess up the bytecode even if I saved without making any changes; it did let me change the basic "size/chance" variables which perfectly replicate 1.6.4 cave generation in 1.7+), and I've seen people say that you can no longer use MCP to decompile 1.12+ on a 32 bit OS.
That aside, none of my mods are Forge mods so that is another issue, although it is possible to get some of them to work with Forge. I know of some other mods that let you change caves, including setting exact values in a configuration file, but they are out of date; it seems like not that many really care about the underground: