Making a Board Game: Triple Axe Hacker

[fizzybobnewt]

Come help me design a strange sort of board game.
My concept for "Triple Axe Hacker", at a glance, revolves around harvesting trees to bigger your corporation, while wantonly disregarding the balance of nature which you are disrupting. However, further in, the game changes drastically; as your meddling with nature makes the playing field dangerous, it becomes an issue of survival and getting your human units entirely out of the area. Even the competitive aspect falls away, as the players are no longer business rivals and must work together to survive.
The thing is, I want the switch to happen naturally,*without*being outlined too explicitly in the rules. I want it to feel like the repercussions of your meddling have gone so far as to, in a way,*break the game itself. Instead of, for instance, separating play into a "profiteering" stage and a "fleeing" stage, which would make you feel like you were acting out a script, I want the decision to abandon the profiteering to feel like your own, and for it to feel like if you'd done better early on such drastic measures might not have been necessary.
Not sure yet how I'm going to manage all that.
As for the more conventional logistics:
In General

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It is thematically a mix between The Lorax, World of Goo, and a little bit of Bone (I don't plan to make money from it). The two players are the Onceler and the Sign Painter, each running a separate corporation at either end of the board. You can both move your units and build structures using goo balls, which are harvested from trees. Also from trees comes "pure goo", which is what you are here to find in the first place; it earns you upgrades, and if you do manage to avoid the final disaster, it is what you need to win the game

Board

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It is a top-down view, a square grid like chess, however, the terrain varies greatly in height, which is why building structures is necessary to get places. There are also some features like lakes and dangerous spikes. Preferably the board would be randomly generated for a session; at least, the tree placements should be. It should also be very big, very many squares, so that the trees can be spaced out while having lots of them and lots of room to explore. The height of a given square would be indicated by some simple visual cue, like color.

Building

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There will be a list of structures you can build, such as bridges, ramps, or ladders, each with a cost in goo balls. There are different types of ball, coming from different types of tree. To place a structure, you need a Handler human in the same square, which is why we have them.

Disaster

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Ghost circles appear. They are invisible, and destroy anything that touches them. As more trees are harvested the frequency of ghost circles gets higher. Maybe there is also another threat, involving monsters or something? Don't really know yet.
I want the ghost circles to not just be a die roll each time you move. I want it to feel like they are already placed and waiting, and it really matters where you step. We could maybe do this the same way as randomly generating the board.

Units

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Vehicles (don't have much in the way of specifics yet.)
Thorn Harvester: harvests trees in adjacent squares. Requires a Driver human; can hold up to three more.
Rose Harvester: is better and more expensive than the Thorn. Can hold up to 5, plus the Driver.
Transport: is faster than the harvesters. Needs a human of any type; can hold one more.
Humans (seems like there should be more types.)
Handler: coerces the goo balls into the shapes you want. You need her when you want to build things. Can also take down structures of ivy goo.
Driver: yup.

Goo

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Ivy: If you pay the Base Goo cost in Ivy, the structure can be taken down later by a Handler and you can recycle the ivy goo.
Dead: If you pay the Base Goo cost in the Dead, the structure is invulnerable to spikes, ghost circles, and monsters if we do end up including those.
Fish: Fish are not found in trees, and in fact are not technically goo balls. They are in lakes. If you spend three of them in addition to a structure's cost, in can hover in the air without support.

Major Questions
How could you avoid the disaster?
How is survival not just a matter of luck?
How do we generate the map?
What are some more unit or goo types that would come in handy?
What about monsters?