I have a suggestion for the Emissary. For now, it's litteraly a sitting duck.
If you compare it to most other arty units, like Badger, Impaler, Firewalker, Recluse, Phantom and other Lance, it's the only one with the Bulkhead (and maybe the cruiser) that need to stop and deploy to fire, but also need to undeploy BEFORE moving.
And the Bulkhead is of big use because it's amphibious and cheap.
Of course, Emissary is not without use. It's the perfect counter for Sling or Badger in opinion, and it's a cute unit.
But when the Impaler come into play, you can't really use the Emissary anymore : when spotted, you need so much time to simply move you are dead, when most other units can immediately run.
Tremor is special because of its firepower, but Emissary is not so powerful in my opinion it need to be counter-balanced in the same way than the Tremor by being a movable tower.
Same for the Impaler : The only real difference between the two is the Emissary has a bit of AOE and the Impaler not. And for the same price and same damage the Impaler don't need to stop to fire or undeploy.
Then here's what I suggest :
When the Emissary move, it can't fire and immediately begin to undeploy its gun. This undeployment has to go to the end : Even if the Emissary stop to move immmediately, it has to finishes the undeployement THEN it can deploy again its gun when immobile.
Then, the Emmissary STILL need to stop and deploy before firing, and still has a period of undeploy-then-redeploy if moving, but at least can do emergency moves, like nearly all the other arties & co.