Could another one be made?
It was too close to important horde territories like Thunder Bluff and Org.
And it’s currently way too chaotic over there. Imagine dying over and over. It’s basically that.
Kind of surprised Mana Bombs have not been used again. They are pretty much nukes in regards to destruction. Maybe we can get the Blood Elves to make another one to blow up the giant sword stuck in our planet. 
Garrosh Mana Bombed Theramore because Theramore was the way through which Varian’s armies invaded Kalimdor. Despite Jaina’s pacifist tendencies, Theramore was a legitimate threat to the Horde. You can see that in the Cataclysm quests in Southern Barrens.
And also because he had a Mana Bomb.
And also because he was evil.
Yes, and many have been made. The first Mana Bomb we saw was actually part of tBC quests in Terokkar Forest.
We also used one in the Broken Shore and a few in Argus.
That said, the Theramore Mana Bomb was particularly powerful because they used Malygos’ Focusing Iris to create it. I doubt another Mana Bomb of that power could be created with the Horde’s current resources.
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Also, from my own understanding, the bomb Garrosh dropped on Theramore was equivelant in horror and destruction to what happened in real life in Japan at the end of WW2. I saw the event as an homage to that very real thing. In that you should get your answer as to why/if they could build another one. Probably. Will they? Every living soul in Azeroth hopes they never do.