Campaign Toybox
The Story: It's pretty easy to read Lord of the Rings as something of an attack on industrialization, with the woodland elves, the gardening hobbits and the trees themselves put against the smoke-belching, land-scouring, gunpowder using Saruman. As always with tropes, they're fun to invert. This doesn't mean you have to make the elves the bad guys (although in fantasy world-altering cataclysms are almost always caused by the side of evil) but it might make it hard for people to see them as anything else.
Picture it: after seeing the works of men constantly destroying their forest homes, plunging their tiny nations into endless wars and using their technology (and the magic they bind it with) to ravage the earth, the elves say enough is enough. One earth-shaking spell later and the world is primoridal once again. Every space of land is covered with trees and grasses higher than any man. Every road uprooted, every city smashed to rubble by angry trees, every dungeon a swamp choked-off with fungus and spores. And the foolish men who only know how to harness the land to their will scattered to the wind.
The survivors would be those who never needed the land anyway (like the dwarfs) and those humans quick enough to realise the elves are their only hope. Suddenly the elves aren't just back but they're in control because they're the only ones who know how to live in this insane world. The humans are like children, unable to travel (horses cannot move through the thick forest, and eagle-charming is something only elves can do), unable to get food (for they lack the bowskills to bring down a bird through the canopy), unable to find shelter (they know not that the curl of the leaf of the giant creeper vine makes a fine bed for any elf). Such children need to be protected, for their own good. Gathered up, set apart from the world until they learn to make their way without hurting themselves. Are the elves the bad guys? Or did they just make a world where they make all the rules? Is there a difference?
Style and Structure: There are two inversions here. First, nature as dominant over the domesticated and technological world. Second, elves not as a force passing away or ephemeral, but front and centre and, by virtue of their suitability for the natural world, running the show. From a human perspective, this can throw things immediately into a post-apocalyptic world where they are slaves to alien invaders. Which is cool but if your play that up too much you may lose a sense of the cause of the whole thing. That is, it's important that whoever the PCs are that they get to learn about their new world and its rules on their own. So don't make the elves too powerful or too in control. Instead, just put the shoe on the other foot: humans are now like elves before; rare, strange, often confused by and always out of place in general society. The oppression will then happen naturally – it doesn't need to be forced into the foreground.
Unless you want to do that, of course.
Obviously, such an apocalypse can be put upon a pre-existing campaign, or starting from scratch, with the usual caveats. The former will have more impact because the players will know intimately what it used to be like – but likewise, they may feel cheated if they have no way of stopping or being involved in such a transformation. A middle ground can be having the change happen in only one area, so if the players get sick of being evolutionary dead-ends they can flee to the land where things are as they were. Such a quest could be the basis of the whole campaign (like the search for Dryland in Waterworld).
PCs and NPCs: Still, being a human in such a world would not be much fun – but could be fun to roleplay. It would also be easy because just as the players would always leap to their typical gaming ideas (“we get some horses for the journey”) their characters would similarly be confused by the new ways of the world. The forest infinite would also provide new ways of looking at other races: any dwarf-elf enmity would grow, as there would be no middle ground between the two, just people with axes and people with wood that must not be axed. Free-running plains races like wolf-men might die out altogether, while heretofore unknown species in your campaign world may suddenly have a chance to take centre stage, because they see so well in the darkness of the forest, or their high Dexterity makes them natural climbers through the branches.
Roles would shift as well. The nature-loving classes – your druids, rangers and barbarians – are now in their element all the time, and get elevated from backwoods bumpkins to humanity's saviors (and best elf-diplomats). Meanwhile thieves who need alleys and mages who need libraries or laboratories are in trouble. As may be priests, because if trees destroy every one of His temples on earth, you may have trouble convincing people your God is really all-powerful.
Mages have an even bigger problem than a lack of spellbooks though. A world so rich in trees will quickly be much richer in oxygen as well (which, by the way, will let lots more creatures grow much larger). An oxygen-rich world is a terrible place to cast fireballs. There may be laws against such things...
Plots and Villains: The great thing about a forest is it makes the world three dimensional: you can find adventure not just by going out but by going up or down. Indeed, the whole forest floor could potentially be considered a dungeon: a wild place ruled by scavengers and giant insects – and all you have to do to get there is miscalculate a jump or trip off a rope-bridge. Dungeons are literally seconds away, all the time. Of course, that may also mean any invasions from below or beneath can happen everywhere at once. It's worth considering how the dark elves/shadowfell/darkforest/netherlands has reacted to all this. Necromancers will also take a while to adjust – no more graveyards. On the other hand, cities reclaimed by forest will be choked with the dead. Fantasy rarely deals with the post-apocalyptic, probably because the necromancers might become unstoppable with so many resources at hand...
Are the elves the bad guys? Maybe. Maybe they set the whole man's-technology-apocalypse up in the first place so they could get their primal world back while at the same time look like they were saving the world. Or maybe it was all an accident and now they're just trying to run things as best they can, with the blindspots common to any species in its own environment. What can be fun is continuing the trope inversion and play them like corporation bosses in a kind of reverse-cyberpunk world (with huge bean-stalks perfectly replacing the skyscrapers). Yes, some humans die but that's the price of regress.
Sources: Waterworld is a ponderous film but it is a good example of humanity trying to adapt to a suddenly completely different environment. For the amazing vertical ecology of forests, try Sean Connery's Medicine Man, or any decent documentary about the rainforest (and look up eutrophication in a biology textbook to see all the problems too much oxygen can cause). For some awesome images of a world reclaimed by nature, check out the documentary series Life After People, but for what might happen if it happened quickly, you'll want to read Alan Moore's Swamp Thing Issue 52 and 53 (collected in the trade Earth to Earth). For plants trying to wipe out humans there's also M. Night Shyamalan's The Happening, and for technological humans outclassed by “elves” with natural know-how there is of course Avatar (imagine, if you will, what would happen to humans who didn't want to become Na'vi, but remained on Pandora). And have a look at Elric for an example of how a higher, elf-like race might deal with humans.
RPGs: The definitive fantasy setting with a twist is of course Dark Sun. There it is too little life that is the problem but look at how that small change changes the entire tone and you'll see what can be done with the infinite forest. The World Tree RPG posits a world inside just one tree but will give you a blueprint on the whole vertical thing. If you want to do cyberpunk in reverse, check out Shadowrun and Cyberpunk 2020 and their friends. For post-apocalyptic fantasy, Earthdawn is your friend (and there the dwarfs are in control because they hid everyone from the Horrors). Of course, most simple of all is to use this idea with D&D and watch what happens as all the overland-only skills and spells become the most used ones of all (note, for example, how powerful Entangle becomes if you're always in a forest!).
