fk is way to hard to use, say if you want to move around the pelvis while keeping your chest in place (kinda like a human spine works), also adding complexity to the animation (say impact tremors or foot contacts to your pelvis), with an fk system you would go litterally nuts, since you can’t isolate the chest or the pelvis, since everything is dependent to the previous control.
I would not use pure FK either for tails, nor necks/head, all ik spline.
You can definitely add a space switch, i do not mind those, alto, again , i tend to do my own locators if i want to lock a control in place, since we usually lock into the “space” that we need, sometimes we even lock it in multiple spaces in the same shot, say you have a character running forward at camera, then it rolls to the ground, i would do 2 different locators for that, and polish up my pelvis based on where it makes the most sense.
One thing that you could add , before the space switch, would be to have extra controls for each of the spine controls, say you have mainSpine ,Pelvis and Chest?, you would also have MainSpine2 Pelvis2 chest 2 , those extras are more important to me then the space switches, i can use these to add extra jiggle to the animation , or try something quick if needed without breaking the main performance. (i don’t like animation layers ,they are way too messy to deal with…
hope it helps.