Yeah, one of the things I liked in old versions was having just one type of planks (not having multiple variants of everything wood, particularly). And I've never cared about the bosses or searching for something 50K blocks away from spawn or whatever. The other annoyance is hunger, though eating to insta-heal isn't much better either.
One issue for me is that I really liked the block model system of newer versions (release 1.8), particularly as a resource pack creator. A ladder looks so much better as a few cuboids than it does as a flat texture, and my models (which I made in a text editor) looked a lot nicer than my textures.
Also, never migrated my account. Are the servers to download the old versions from the old launcher even still up?
Minetest could be a solution here, but it seems like most Minetest games are either following new MC's footsteps or are doing something completely different. At least I've never played one that made me want to keep going, something good enough to start my own thing with (I would like chaining sticky pistons or similar things that are powerful in single-player, blocks that look cool but offer specific benefits like an iron grate floor/ceiling).
Parallel timeline mods are interesting, though I am not having luck with trying them thus far and I also doubt the modding tools are there enough especially for me who doesn't want to code in Java. I could also see it interesting if there were an easy way to just disable a large amount of blocks/items/mobs etc and then just add in new stuff... maybe even with data packs especially for this sort of thing.
I am thinking about game mechanics that interact (has anyone tried liquid-like gravel/coal piles yet?) or that just connect simply/are instant (rather than high-throughput automation). Or different systems for healing/buffs/food. Maybe alternate tools/transportation/skybridges etc.
EDIT: So they really added data packs without the ability to make "true" blocks/items (instead still dependent on entities and commands, data overridden not data driven), huh? Guess I shouldn't be surprised.