(edit: Okay, this is more like a major major expansion than a DLC... But I still want it. x_X)
"Echoes of the Great Tarnalin Forest"
Characters:
--Sensoths as playable race.
--New body types (Slim / Heavy / Muscular) and a height scale.
--New customization options. (Hair, Face, Markings, etc.)
Combat:
--Dual Wielding pistols, charge blades and other 1-handed weapons.
--New melee weapons. (Two Handed) Swords and Polearms, both metal and "charge" types. There would be different design themes for the weapons based on each playable culture... Irathient polearms would be more tribal (feathers, carvings in wood, paint), castithan ones would probably be a light silver with light blue patterns.
Crafting:
--Gathering Skills--
Foraging - Collect plants and other biological resources.
Scavenging - Collect metals, generators and other forms of technology.
--Production Skills--
(Requires Foraging)
Scientist - Creates stims, biological grenades, infector mods, biomagnetic gun mods.
Provisioner - Creates food that you can set up for yourself and others. Eating will give long lasting buffs to different stats such as run speed, health regen, etc.
(Requires Scavenging)
Weaponsmith - Creates most weapons and most mods.
Mechanic - Upgrade, tweak and customize vehicles (for performance, etc.), also deploys a (non-overpowered) turret gun for a limited time after leaving your vehicle.
World:
--New storyline. Subjugated sensoths rise up against their cruel castithan masters with the help of concerned irathients AND you. (This would be an isolated conflict, not something that's affecting the whole world. Something like a rogue faction of castithans.) Maybe Cass Ducar returns for the story.
--New area: Terraformed Forest in which the new storyline takes place, not sure where on the map it would be... We'd probably have to "quick travel" there. There would be one or more rather large castithan settlements, and the irathient NPCs would be hiding out in the woods, maybe in caves.
--In addition to the rogue castithans, the woods are filled with votan beasts, many of which have been mutated in ways similar to the hellbugs. There could be a new type of invasion, with these alien / mutant beasts just pouring out of the woods.