When a game is released, its assets tend to be made so that CPU and GPU are stressed equally. If you have more room on the CPU, you add more different assets, if you have more room on the GPU, then you add more vertices, shaders, larger textures etc.
But as the game ages, advances in new GPUs tend to be much faster than in new CPUs, so it's only normal that older games will be limited by CPU.
Really? Some of the ideas in your OpenMW Trello sounded good. Then again, I wouldn't know which would actually constitute a significant performance increase. That, and they haven't been revisited since 2015...
'Multithreaded skinning', 'Particle rewrite' and 'Balanced scene graph' would have helped the Cull thread, if that was the bottleneck. Or we could improve the camera threading so that water reflections can be culled in another thread. Again, only helps if culling is a bottleneck.
'Shader lighting optimizations' would help the GPU thread, if it was the bottleneck, and if shaders are even enabled.
But from what I'm seeing in this thread, the Draw thread seems to be the bottleneck for most people, and I don't have many ideas for that.
It's possible that opting for Vertex Array Objects instead of display lists (to be released with OSG 3.6) would help with AMD GPU's (I know it doesn't help for Nvidia, display lists are still king there).
Really we'd need some form of batching to properly improve perforance with OpenGL, only that's a massive PITA for Morrowind because static/non-static objects aren't designated in any way, there are many lights that can move at any time, objects can be disabled by scripts, etc. It's possible to do this, just a lot of work and there would probably be some additional stuttering and lighting glitches, as in the Ogre3D version. Couple this with the fact that such work will be obsolete when Vulkan comes around and you can see why I'm not keen on trying.