OMFG i absolutely love this idea for a civ! ive been dreaming of a time when someone would create a plant based civ!
now for my ideas
muhuhahaha
1. Unique improvements:
The workers should get a few unique improvements and be unable to build any of the generic improvement. if the plant civ is destroyed so too do their unique improvements. ie for a few ideas:
Fruiting Grove: +1 food, build only in forests, turns grain resources (rice, wheat and corn) into Fruit resource.
Mycorr Field: +1 hammer, buildable only in forests, may spawn a 'Mycorr guardian' when enemies come nearby, weak, no upkeep cost units that explode on death poisoning the enemy akin to the worker explosions. (Mycorrhizae is the scientific word for the 'roots' of fungus. it is from these mycorrhizae that actual mushroom fruiting bodies grow from)
Preserve: +1 Happy, +0.5 health
2. Palace Effect:
Root spires grow from below ground at the perimeter of the civs culture, these roots of the giant trees can be magically crafted to grow up out of the ground to form a magical barrier against the spread of hell terrain, desert and ice. Root Spires should automatically occure on the perimiter of the civs cultural borders, similar to the Great Wall.
Palace Mana: Nature, Creation, water
-20% war wearyness (these plants live to remove humanoid races from erebus)
+1 free specialist from the unique improvements listed above (this could be one of the only ways that the Plants could get specialists, by building improvements)
3. World Spells:
Whispers of the Forest: reveals all forest tiles in the world to the player regardless of who owns them
The Rooting / Entrench: all plant units gain the 'Rooted' promotion which gives them +x defence strength, +y% heal rate increase, held, and a spell that allows them to turn into a Giant Tree city if not already in a city or within 2 squares of another friendly city (ignores enemy culture or distance to enemy cities). lasts for z turns.
Overgrowth 1: Each turn 'living forests' (the ones that damage enemies) grow out from your cultural bordersfor x turns. ie:
Overgrowth 2: All units get the 'Rapid Growth' promotion giving them nature and creation mana affinity +1 for x turns. but at the end of the spell all units get the 'exhausted' promotion, making them held for a period of time.
Overgrowth 3: All cities get a special Building which increases all yeilds by 50%, as well as giving +x ammount of happiness and health equivilent to the number of forests in the fat cross divided by 4.
4. Unique spells:
Nature:
1. Ironbark Skin: adds the Ironbark Skin promotion to units, +x% defence strength,
2. Forest Vengence: Causes enemies in forests adjacent to the caster to take physical and poison damage.
3. Awaken Hesperides: cast in a city. Destroy city, summon a 'weak' avatar Hesperides unit with nature and creation affinity, on top of whatever other awesome avatar abilities. the Hesperides should get one promotion for each building that was in the city, and a spell that allows them to re-plant themselves, restoring the city to what it was before, with each promotion adding that particular building back to the city. when cast the caster cannot cast any other spells until the Hesperides replants itself, and the caster is forced to move with the Hesperides.
Creation:
1. Animate Forest: turns the casters square into Living Forest (the one that damages stuff, not like Bloom) 3 turn cooldown period.
2. Barren to Fertile: Transforms Desert tundra or Ice terrain directly into Grassland.
3. Create Wood Dragon: The Plants having encountered dragons such as Abashi decided to adapt to such an enemy, and t ofight fire with fire. metaphorically. they devised magic to animate a dragon from scraps of Deadwood. a gigantic construct held together by wisps drawing energy from the plane of creation and given life, more specifically the life mind and a purpose of the caster of the spell. this would be a powerful world unit summon, which kills the caster (a small price to pay methinks)
Water:
1. Create Wetland: turns the tile into a single tile freshwater lake. cannot be cast next to rivers, ocean or other freshwater, teleports caster to another tile adjcaent to the lake. 3 turn cooldown period.
2. Summon Waterweed: castable next to coast only. Summons a Waterweed unit, similar to the Kraken. Imagine a Kraken made entirely of seaweed
3. Earthblood Tap: castable in cities only. causes the Great Tree to tab into the Worlds Earthblood, Water, and drink its energies. all units within 2 squares of the tile get +25% heal rate, the city generates +2 health and +2 Happiness while the caster remains in the city.
5. Water Travel
being plants, they would be fairly against travelling across water. this however is a big issue in civ, especially on island maps.
One way they could counter this is by having aquatic plantlife act as their navy. im thinking Giant Lilly Transports, Floating carnivorous plants, Seaweed submarines.... ok im scraping the bottom of the barrel now haha.
EDIT: i really like this idea:
# Gaea Terrain
* Gaea Terrain is an oddity in Erebus; it is an ecosystem that is so vibrant, adaptive, and inexhaustible, that it can resist the destructive touch of fire, and the corrupting presence of Hell Terrain. Units that travel through these strange ecosystems must beware, as the creatures that inhabit these areas are just as bizarre.
* Gaea Terrain converts forests, jungles, and marshes within the borders into Eternal Forests. Eternal Forests are resistant to the effects of Smoke and Fire, and non-Plant civ units that travel through them have a chance of being afflicted with the Diseased or Plagued promotions. Eternal Forests disallow the construction of many mundane improvements, like lumbermills and roads, as the forests are too wild too tame. Furthermore, the Woodsman line of promotions do not apply to Eternal Forests.
* The work time and cutting time are higher than that of Ancient Forests.
* Eternal Forests will override Ancient Forests on all occasions possible.
* Eternal Forests can exist only within the borders of the Plant civ. If an Eternal Forest exists outside of their cultural border, it will disappear over time.
* This feature makes the Plant civ act as a Good civilization in regards to Hell Terrain spread within their borders, i.e. it is currently impossible for Hell Terrain to enter their borders. Additionally, Hell Terrain within their borders will convert back to natural terrain after a while passes.
+25% while on grassland or plains - the most adaptive and useful terrains for a Geomancer - as well as the ability to raise or flatten a tile, making it flatlands or a hill.
I asked the author, I'm waiting for an answer...
As for the unique terrain, yeah, I'll treat that with UNIT_666 post.
is that a 'its amazing how many good ideas you have there' amazing, or 'its amazing how much rubbish you can type' amazing 
