Block display entities should have block state and interpolation capabilities
Summary
Currently there is no way to modify a block display mob extensively without vanilla commands. The only capability that exists right now is a displaytransformation mechanic, but it lacks an attribute for interpolation. Additionally this feature is pretty useless, considering you can't set the block display to actually represent a block.
Usage
The interpolation capabilities would be used for animation, and setting the block state would make block display mobs actually look like a block.
Value
With these features you can easily animate at a lower level without having to go through programs like blockbench and setting up multiple different configs. Currently the only way to use block displays like this is through commands, but that spams the console with messages like (Ex: "[00:06:45 INFO]: Modified entity data of Block Display").
Priority
I believe this shouldn't be difficult to implement, and I believe it provides a pretty high value, therefore I think it's fair to put a decent priority on this feature.
Implementation
It's a simple case of modifying NBT, I think it's pretty self explanatory. Spigot even has a setBlock method for the BlockDisplay entity that directly sets the displayed block. Interpolation should be easily added the same way you added capabilities to change the transformation with displaytransformation.