> The graphics were secondary to all of the other things I mentioned
IMO, not exactly. At the time of the creation of Minecraft, "ugly" voxel graphics were a necessary technical evil in order to allow the world to be fully editable.
And then, when you start from that, the only aesthetically consistent choice for textures is to stick to retro-style low resolution. photo-realistic high-res textures don't feel entirely right.
> Pulling off the procedural generation and motivating the creativity is what made this masterful. Those were difficult features to program
I don't think they are; it's a matter of knowing that algorithms for noise generation that are both suitable for landscapes/caves and computationally efficient (perlin/simplex noise) do exist. The procgen trick itself exited for a long time before MC; Elite is famously known to have taken advantage of it, and roguelikes were using it before that.
> It was the _combination_ of all of those things with a challenging environment and mechanics that motivate you to explore those features!
To me the holy grail in not-goal-oriented true-open-world sandbox games such as MC is to ascend to the infinite game (which remembers that one source of inspiration for MC was Infiniminer). A low-hanging fruit are the "creative" players, that is players that are satisfied with building huge cities (as one of the screenshots in TFA shows) or replicas of the Enterprise, Star Destroyers etc. Those tasks can in practice be infinite. If that's not enough, you can add other infinite axis, like "red stone".
It's more difficult to achieve it with RPG-oriented players, because traditional MMORPG have a leveling mechanic that necessary has a maximum; lore and stories are also finite by default (this could change with generative AIs). Unless you let players create them. But you have to move to "true" role-playing to achieve that, like you see in also decades-old MUD games - MUD games that somewhat share with MC the property of being easily extendable because text is a much cheaper asset than audio/video/3d models. But role-playing somewhat assumes multi-player.
In single-player, a possible way out is to horizontal progression - that is, rather than having a higher and higher level up path, you have players to choose between mutually-exclusive options (the choice between tank/mage/dps in MOBAs is an example of this). In a game like MC you can have players choose between attributes that helps with farming/fighting/mining/building. If you pile up those attributes, you can get a lot of player character diversity for cheap thanks to combination explosion - much like you get a lot of different-looking characters with a relatively small amount of textures for clothing. With this device players are encouraged to play multiple characters specialized in various tasks. It at least should create some form of trade between them.
[1] https://en.wikipedia.org/wiki/Multi-user_dungeon and https://www.mudconnect.com/