Currently, transforming in to something with a bigger health pool means you have to regenerate in to that pool of health. It's very annoying, and severely limits the combat utility of transformations.

What I propose, is instead of just setting your max health and being done with it, you make it also set your current health to the same ratio.

For this example, you have 10 hearts normally, and your new form has 20 hearts
If you're at full health when transforming, you'd end up having 20 filled hearts of health.
If you had taken 1 heart of damage before transforming, you will transform with 18 full hearts of health, with 2 empty.
Similarly, if you transform back to normal after having taken 1 heart of damage, you will end up missing half a heart of human health.

I proposed this a while ago on the Witchery: Resurrected discord, and Ashley seemed interested, but it's unlikely to come to 1.12 if ever implemented.