D-E-V-O-L-U-T-I-O-N = Alright, so you got all the basics covered. So some further ideas for optimization:
1] How high is the polygon count / texture resolution on each Tentacle. You might be able to optimize them when they aren't visible or at a distance away.
2] When a tentacle travels through the water, is the water just a textured surface, or is it an actual mesh that deforms as the tentacle passes through it?
3] I know the game pretty heavily relies on fog. If you totally disabled the fog, does it improve performance? If so, then try to look into optimizing the fog in some fashion.
4] How does the game handle darkness? Is it merely the absence of a light source or is it using soft-shadows to instead?
5] When the AI is not directly seeking out the player, how expensive is their pathfinding? If it is quite expensive, then you might consider patrol routes instead of them wandering randomly. That way you could also decide their specific path ahead of time.
6] Are you performing physic calls on the grass, trees, shrubs, etc... Witcher 3 had a very well-optimized way of doing that (mostly by disabling the collision on the plants, as they'd clip right through walls when it was windy)?
7] When you have weather effects, are they particles, sprites, etc...? Like rain drops...
8] Since the later builds allows for visual clothing customization, are all the meshes that can potentially be worn, pre-rendered and thus taking up valuable resources (ie RAM)? I ask because Witcher 3 would pre-render everything in your inventory, so you could quickly view the mesh of any object?
9] How high are the polygon of non-essential map elements? Rocks, fences, trees, etc...? Services like SpeedTree might optimize their meshes, without too much loss of quality.
10] Same idea as 8, but now for the weapon parts? Or items in your inventory.
11] Since you can have belly expansion with moving tentacles and bulging of eggs, do these objects have a high polygon count / textures of their own?
12] For shooting mechanics, is it using Tracers or simulated bullets + trajectories?
13] When an enemy detects the player, do they always know where you are, or do they have to maintain visual conal lock?
14] How often do you use double-sided meshes? These are like when you have an armor vest, that has the outer section and the inner layer rendered; most games use double-sided meshes sparingly, as appropriate.
15] When you are in 1st person, is the 3rd person mesh fully rendered? Similarly, in 3rd person, is the 1st person mesh fully rendered?
16] What does the enemy "see" when they shoot at you? Is it a fully rendered object, or a simple object, with a low polygon count?
17] Is all the items in a chests, looted or otherwise, rendered meshes, or only rendered when removed from the chest?
Probably have more ideas later.