I'd love a Terminator based mod, something heavily firearm focused akin to The Reckoning with Futuristic weapons. The story would be a post Skynet war world, where humans have settled down and started to rebuild while the Machines now leaderless have hidden themselves in the more inhospitable portions of the world. Both have splintered into factions and are frequently fighting against each other.
Naturally, the player could choose to play as a human or a terminator, with significantly different mechanics for each. I'd leave it to the modder to figure out some other mechanics (I want the two to end up very different in the end so they're both worth playing as) but one thing that comes to mind is equipment. While humans start out the much weaker of the two and are overall more fragile they have access to a much wider range of armors that'll boost them up to the Terminator's level provided they have enough money. On the other hand Terminators start out stronger statistically and are able to take and block more damage, they have cosmetic slots that just change their general appearance such as wearing a leather jacket and jeans but don't improve them statistically. Terminators have to salvage from battles with enemy Terminators better body parts which are almost always heavily damaged and thus only very marginally better than what they start with. Either that or by increasing their level and crafting skills find the materials, blueprints, and tools to make better and better body parts in a much slower progression that likely would take the player all over the map.