I've come up with an idea to make the final boss fight more exciting and that is a unique three way battle between the Iron Horde, Defenders of Azeroth, and the Burning Legion.
I think the raid group should fight these two in an open battlefield. Two tanks, the main one on Archimonde, the other keeping Grom busy.
Grom fights similar to Garrosh but without the special abilities, the tank simply has to keep him away from the other raid members. Archimonde on the other hand is a killer and the true focal point of the encounter, he does everything and anything to keep the DPS on their toes and has a lot health.
To counter this some of the DPS must defend Iron Horde soldiers from demons as they construct war machines that will do consistent damage to Archimonde. There is a limit to how many can be built and when that limit is reached Archimonde will summon one of his doomlords in this order. (Kaz'rogal, Kruul, and Kazzak.) These guys all have unique abilities and must be taken down quickly by all the DPS.
At 75% 50% and 25%, Archimonde will cast a spell to destroy all the siege equipment, but in this moment one of the heroes (Thrall, Yrel, etc) will stun him. Each time the DPS has to burn Grom down but they must be careful not to kill him before Archimonde as that will initiate a hardmode where Archimonde is buffed up due to feeling his victory is assured, he'll go crazy there.
Grom and Archimonde can interact in this fight however, the tank can kite Grom into one of Archimonde's AOE blasts which will damage Grom but he will retaliate and attack Archimonde with a huge cleave.
Archimonde is a dangerous target. He has a lot of plague based attacks and a Finger of Death spell which will focus on a player and strike at their location if not avoided its an instant kill, and it is very quick.
When Archimonde is killed, he'll explode in a huge AOE. Someone will shout out for Grom to yield but he'll scream Lok'tar Ogar! and keep fighting until the raid finishes him off. When this happens Grom should be at 10% health if the DPS has gotten him correctly.