Thats the short version, now lemme put forth some of my ideas for such a mod with details and such!
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Lets imagine that you start in a desert, on a planet devoid of almost any water, with little to no trees or plant life. All you have is your tiny water hole, which you must be careful not to drain (assuming you use a realistic mod like Water as a Resource). You search far and wide to find trees of any kind, harvesting them in the hopes of finding seeds. If possible a percentage chance to find seeds when harvesting wild trees. You craft a shovel, used on grass to dig up grass seed making the land it was used on into barren rocky landscape.
Now, you've got your seeds, a small amount of grass and tree seedlings. You build a greenhouse for the grass, which takes electricity and water as well as a supply of dirt (crafted by mixing iron ore, coal ore, and rock or dug up with the same method of digging up grass). Now you can grow more grass in this greenhouse, self contained 6x6 wide building which will produce grass seed for you over time.
For your trees, you till the land, fertilize it, manage it carefully, turning that spot into a farm. After which, you can place down a tree farm building, which can not be placed on any tiles except your specially treated tiles. Within this building, trees will grow. Producing seeds, while consuming the same things as above in larger amounts. I'd say this building is probably 12x12 tiles in size with transparent texture to see the ground below, basically just a fenced in area with some mini pipes around and some various tools and such laying around the place, maybe even mini belts inside. Who knows, its just a tree farm though, it probably might be lag inducing if you try to code it to place random trees inside instead of just giving it some trees in the graphic but I dunno since I'm not good with coding and such. I'd like it to produce seeds at a rate of how many adult trees are inside, but I'm not sure how possible this is. This building produces negative pollution, removing pollution in the tile its in and 1/3rd the same rate in the surrounding chunks.
So, you have a steady supply of seeds, and you've likely by now researched the machines needed to change the land. One of these machines, looks like a bore into the ground with which you connect it to a pipe network to pump in water. This machine will start to turn all low fertility tiles nearby into mud, the range increasing over running time at a diminishing rate. Within 5 minutes, it'll have a range of 5 tiles. Within 30 minutes, a range of 10 tiles, 2 hours, a range of 20 tiles, so on so forth. Some similar rate or some such as it expands further over time. Any tiles above a certain fertility rating will remain unchanged. Another machine, will focus on the fertility range that the mud has, and tiles between certain fertilities. This will turn the mud tiles into proper dirt, and change landfill tiles into dirt as well. Working with the same premise as the other machine with an exapanding range system.
Now you place a very tall machine down which will pull grass and tree seeds upwards to the top and spit them out over a certain range randomly. The grass will grow over tiles with a high enough fertility, and the trees will grow into normal wild trees. This tower can be set to grass or tree mode. In grass mode, it continues to operate till the grass level reaches your desired level, 50%, 75%, 90%, or 100%. Tree mode, same thing, but with 20%, 40%, 60%, 80% as the chosen options. These trees will be wild regular trees, so mods that control the natural spread of trees can take over.
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Lets not forget, some players will want to change the land to something else. So the buildings that change the land to mud, should have options in them to change the land from one thing to another. Each requiring special things, such as energy, supplies, time. Changing fertile dirt to dry desert would suck up the fluid from the land, dirty water mixed with all kinds of minerals and such which will require processing to clean it into usable water.
Pollution should also damage the landscape at certain levels. When pollution reaches high enough, trees die, grass dies, land fertility begins to decrease till it turns down into different levels of damaged dirt or barren landscape which is just dry cracked dirt with zero fertility.
Placing floor tiles on top of dirt, changes the fertility of the land beneath down as well unless its already a land tile with low to no fertility. If possible, requires being down for a certain amount of time before that change happens. Same for structures like assemblers. Anything that would normally block light.
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Lets not forget something important though! Water.
If water is a limited resource, you'll need a way to make basins for it to collect in and fill up for your pumps. With the mod "Water as a Resource", larger bodies of water refill their water content faster than smaller bodies due to their surface area. So. How do we make a new lake?
Lets make an excavation floor tile. Much like the hazard floor, but with orange lines instead of yellow (with red for deep water). You place this tile down over every tile you want to turn into a water bed. Nothing can be built on top of these tiles. Any structure on top of them is immediately destroyed and dropped as an item (unless its possible to prevent the game from letting you place these under buildings which would be far better). Now, you place an excavation machine next to these tiles and it will scan all connected tiles and calculate out how much time and resources it will take to dig up this area, chunking out the material such as dirt and other materials it finds in there out onto the stockpile in its inventory for you to pull out with inserters and belts or whatever you plan to use. Cannot be used to collect resources faster. You'll get lots of dirt this way, possibly random bits of iron or coal ore, lots of stone too, random bits of wood from tree roots if proper land type is being dug up. You can use yellow hazard tiles, to mark a large area, then fill in random bits of that with orange and red to make a random lake of sorts. If you place the excavator on yellow hazard tiles, it will check all connected tiles (yellow, orange, red) and calculate out the area and such for every tile. Yellow is just to connect bits in between orange if you want to make multiple ponds, for example with one excavation.
This process turns the land into "water bed", which is empty of water. You must now fill it with water by placing a pump next to it to pump water in. Filling it with water. If you have the water as resource mod, it will inherit your values for how much each tile has in it, to fill. Perhaps even compatibility could be added to turn these water beds into water areas that the mod uses already for refilling purposes with other compatibility to have that mod turn empty lakes into water beds. Water beds cannot be built on, but can be walked across at a slower rate. When filling a water bed, it fills deep water first. At 15%, it becomes shallow water inside the deep water bed portion, walking speed decreased 50%. At 30% filled, deep water bed looks like normal water. Walking speed is set to whatever water walking speed is (assuming you have a mod for that). 50% filled, deep water bed becomes deep water, while water bed becomes shallow water. 80% filled, the entire water bed is filled to be like an undrained water area. Its not yet ready for a pump to be put on it till it hits 100%, at which point it becomes normal map water.
You've now created a lake! Whether this lake uses base game logic of being unwalkable and infinite, or real world logic of having reduced walking speed and being drainable, you now have a place to fish! Assuming you can find a way to get fish in there.
Boy all that excavation though, you've probably got tons of worthless material like dirt, which could be used to make land fill, fertilizer, or whatever. Lets hope you got a warehouse to store all that in!
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Me personally, I like realism. So I'd like the mod to use as much scientific accuracy as possible, and to be well balanced for long gameplay. No converting the entire map into a paradise in a single day. This should be a project that takes weeks of gameplay just to convert your starter area into a paradise with tons of dedicated effort. Obviously mod settings could be added to increase or decrease the speed to make shorter or longer gameplay.
Right now, this is just a bit of an idea. I'd love to see a complex system of terraforming though to change land tiles from one thing to another. Including landfill tiles which, with this mod might end up more complex to produce even.