Still working on the in-world representation of tools. I've decided that they need to be simulated on their own "layer," that is to say, a given space can contain both a tool or other object and more substantial world simulation elements like water and gas.
What do I mean by a tool? Tools are objects the player can interact with, place in inventory, but also change the world to his liking. Changing the game environment is a major theme of the game; the pitons and ropes you place to get around don't go away over time unless something removes them. Since you can only carry so much at a time, exploring is an iterative process of placing equipment, over multiple expeditions, letting you get further each time.
One thing I'm worried about is that the process of traveling back and forth in the game world, between base for resupplying and the edge of exploration, could end up being boring. One solution to this problem I'm considering is the use of player-positioned checkpoints. The idea is, the player could obtain "flags" that he could place in the field. After planting a flag, the idea is to then "validate" the flag, by traveling back to the base without placing any more equipment. If this is done, the flag is validated, and the player can then warp from the flag to the base at the cost of the amount of game time it took to make the trip. Likewise, the return trip can also be validated for reverse travel.
This system isn't perfect; after a flag has been validated going one way or the other, it is possible for the terrain between the two to change and make the route impassible. This could be partially solved by keeping a list of the cells traversed in the route, and upon teleporting checking each of them to see if they've been modified since the route was validated.