My Idea stems from many summons, etc. from past Final Fantasy's that rarely get any spotlight. Most of these are good summons, etc. however in this game they are Primals.
My idea would be basically, that there are good primals in the world of Eorzea. They could either be ancient protectors of past cities, or have unique situation and connection with the gods of Eorzea.
One such Primal could be Alexander from FF9. Which was summoned to protect Alexandria and fought off Bahamut. These battles could range to massive fights, where you fight with the Good Primal to protect it against hoards of Beastmen, including that beast tribes "Hero", and their primal. Could consist of even an alliance of 2 parties big.
The Lore again, could be that they are direct protectors of the Gods themselves, which would fit perfectly with Alexander, especially with how big he usually is shown to be.
It could even be focused on an expansion where the Beastmen hoards come together, as a full onslaught war against the other inhabitants of Eorzea.
Just my idea behind what could possibly be an awesome expansion to FFXIV:ARR with alot of lore, extra content, and to pull in Summons from past FF's that are rarely seen like from FF9, but with a twist.
Edit: Re touched up my post as some of it wasn't explained properly.