Minetest Game is predominantly enriched by the multiplayer component. Therefore, any feature inclusions will mostly focus on that, as there is zero PVE aspect introduced in the Minetest Game, and mob API support is a long ways off + requires a considerable amount of time, dedication and effort. While I will touch on the ways PVE aspects can be introduced (some form of inherent danger providing a sense of challenge without introducing an entirely new mob system), there is no denying that the multiplayer scene is just as important - one look at the server list and the predominantly used gamemode will support my view.
Player VS Player additions
Spoiler
2. Player's Nametags should have a limited range. Nametags should not be seen through walls, and sneaking should completely hide your nametag. This fulfills several purposes: one, it declutters the screen from players that are way too far away from you. Two, it makes it more difficult to find each other, which is especially important if you're hunting someone down or stalking them - there should be challenge to that aspect of gameplay. Three, sneaking should allow you a sense of security, and will introduce a large incentive to exploit the darkness to the fullest.
3. As a result of 2, a visual indication of a sneaking player should be introduced, such as a crouching stance.
4. Visible armor to support the sentiment expressed in 1.
Spoiler
2. Darkness poses inherent danger as-is - unlit areas are harder to navigate, and it's easier to die due to fall damage, especiall underground. As a result, it should be considered to make torches non-permanent. This aspect could deviate from Minecraft considerably by making it so torches only last a certain time before they exhaust their use. Since flint and steel already exists, perhaps torches are unignited when first placed, and flint+steel must be used to ignite them. Duration for which the torches are available should last a considerable amount of time, but more permanent light source solutions would be encouraged.
3. Radiant damage could be introduced for lava/mese crystals, or even to create harmful mushrooms, spores, gases underground you could bump into that you would have to navigate around or break.
4. Introduce cave-ins in deeper levels, causing blocks to fall and potentially damage you unless you set up supports using blocks that aren't stone (wood?). (More brainstorming for this one needed)
5. Chopping a tree block should bring down the whole tree, perhaps introducing gravity making the tree fall. Introduces danger to chopping trees as falling trees may result in damage sustained to the player. (Possibly a complex feature, though we could use Oil_boi_'s efforts as a baseline)
6. Finite water/lava sources. This change is done both for PVP and PVE sake. In Minecraft, a lava source block can be used to cheese the majority of the game's challenges - lava can be dumped on the mobs to cause massive amounts of damage to them, and their pathfinding always leads them into the lava. In this regard, Lava is also incredibly powerful as a weapon in PVP, leading to large lava floods creating massive area denial for other players. It's also not terribly difficult to do, and you can always re-obtain the same lava source, making it an infinitely useful weapon. Ensuring both water and lava is a finite resource reduces the liquid exploitation, once again moves away from Minecraft's mechanics and creates a much more dynamic and interesting liquid system overall.
7. Water pushes items, lava destroys items, and flowing water/lava should push players around. This would reduce the amount of "waterfall climbing", which is once again a staple Minecraft feature that I'm not particularly fond of.
Feel free to reply with your suggestions and ideas, the endgoal with this is complexity and depth introduced from simple to understand and implement mechanics where applicable.