Currently in spigot the CreatureSpawner object has the method,

.setSpawnedType(EntityType type)

This is good however you can not edit the entity's meta data like you can do to already living entities.

It would be useful if spigot had a method, 

.setSpawnedEntity(Entity entity)

In vanilla MC you can set custom NBT data to mobs in Spawners such as Armor Stands, so it would be useful if the spigot API had this support as well.

Uses could be for Custom Blocks, which is now more appealing in 1.14, which use an Armor Stand in a Spawner with custom modeled item on it's head. This feature would be very useful for me and I would love it to be added.