I thought a lot about it. The problem is, to make the implementation in code as easy as possible, it must be flexible, but don't eat too much computer-resources and should make a high usage-factor, which doesn't destroy the game and makes too much micro management. Much things. This is how it may work:
First you need a "robot station". It is works like a wooden chest but has an orientation (maybe a 2x2 fields?). It should be available from beginning (or very early). You need to built also "walkers" and put it into the station, the station sends them automatically out (until the limit is reached, see down). Eventually you have a own slot for placing the robots, so they are not taken out by an inserter.
Once a walker is controlled by a station, it belongs always to this station until it is taken down by the player or destroyed.
The station scans a limited area "in spirals" (the station is a lua-program which creates programs for the robots). When there is an item on the ground it tells a free walker to run to this point, pick it up and go back to the station. When the item is removed meanwhile, the walker returns without item. It may be disturbing, because "on the ground" are also items at the end of a belt or items which picked out of a factory by an inserter and lay now on the ground. But this can also be used as strategy to collect the output of some factories into one chest (= robot station works like a chest).
The scan prefers first to pick up lose items, before it picks up resources. This is also the way you can control the box: Place it near a forest, and it begins to cut it down. Place it near a miner and the robots collect the output.
The robot station can be built only once or maybe twice(?). One robot station can control up to 10 or 20 robots. When you click on the station you see, how many robots it controls currently.
The walker robot is like a walking burner inserter with an optional slot for axes (they have an own slot for that, when you put axes into the station, before you put in robots, they will be equipped with them automatically). The robot station can upload a program into any walker-robot, when he has finished the former program. When clicking on a robot you may see the current program... for this job it is simple: go to point X/Y, pick up an item (or cut an item and pick it up), go to direction X/Y (which is the station entry), place item, ready.
The robots works slower than the player. They can take only one item. For big trees, which consists of two wood items, one is dropped to the ground and can be collected later.
They must be filled with coal. This can be made like later with the locomotive: An inserter may be placed directly to the front of the robot station and fills coal into the robots when they unload their item into the robot station. They don't need much coal, so this is normally more than enough to refill one or two coals per walk.
When the coal is empty, the robots may not stop completely, they walk very, very slow. This prevents the player from searching hidden robots. When a robot is blocked, it begins to take down everything which is in the way; doesn't destroy it, it just takes it down like the player does, and puts it on the ground, so it will be collected later. The axe gets broken with the time - as the players axe so you need much axes. They can be refilled like the coal.
When you move the robot station: As long as it isn't placed, the robots try to finish their program, when finished, they don't move anymore (player can collect them and the dropped items on the ground, but remember, when picked up, the robots forget to which station they belong). When you place the station, it begins to scan again and sends the robots to the new place, even if the distance is very big.
Later on you may research updates. More robots, more boxes, faster, can be equipped with steel-axe like player. A wider area is scanned. Maybe filters can be set, so that they are useful to collect small resource fields. They can be equipped with weapons, ammunition, like the player. They can protect the player, may be sent out to destroy creeper villages... You can research more complicated programs for the station. The robots know more commands and have a larger memory...
Usage:
- speed up built up time
- fun to watch them
- learn how to fill up locomotive/car later in the game
- cutting down big forests which take the player too much time to cut down by himself.
- get resources from mines without belt or dig resource fields instead of miners.
- mine very small fields or fields which contains two resources.
- get resources from inserters or end of belts
- cleanup area, that was "wasted" because you picked up a full chest and your inventory was already full.
- much better usage for filter-/smart inserters when used to collect items from factories or end of belts.
- much better usage for old burner inserters, and axes.
- with two robot stations and some updates: Program one to plant trees and the other to cut them?
- completly new types of gameplay, when the robot station is programmed differently...
- Maybe usage for some scenario, where you have large fields of free laying items?
- I don't know, if they should be attacked by the creepers? But when so, they can be used to protect the player without weapons...
