So, this idea has been floating around in my head for some time now, and I think it's time to bring it to the community. Since the story of FFXI is over I'll share where I think SE could go from here with the world.

So this new game would take place years into the future, maybe 20-30 years after the end of FFXI. I think it should allow players to import their character from FFXI as like their teacher or mentor NPC for their new character. This mentor can later become like a support NPC (kinda like how original FFXIV had a support NPC that was replaced by the chocobo in ARR). However, due to the time since the end of FFXI your support npc "mentor" won't be as powerful as they once were. Sure they'll sport the iconic weapons and gear from whichever job you set them to but it will be scaled down.

Story:
You can start out the story as such:
Vana'diel once on the brink of destruction was saved by a valiant hero who took up the call. Time passed and now that hero must train the hero of the new generation. The world has been at peace since the hero's time, but something is stirring in the darkness. A new threat is emerging and it is up to you to take on the mantle of your master's legacy.

Missions/Quests:
I think story missions should be similarly set up, but have a big chunk of the early missions be from your mentor, and then mid to late level vanilla missions should be for your chosen city. As far as starting point, you should start at a point outside your chosen city training with your master. As part of your training he/she will have you register with your chosen city so that when the story picks up the city will have a reason to call on you.

I think one questline that will likely need a major change is the first tier artifact gear. This should likely be from your master when you hit the threshold for the gear. Have the master pass on the gear from teacher to student for completing various tasks that are job specific, similar but not exactly like FFXIV's AF quests.

Obviously this isn't a very detailed beginning, but FF isn't my property so I'm only suggesting a starting point.

I think mechanics should work much the same as the current game to appeal to those who spent so much time in the world they loved. While FFXIV is great in its own rights there is a hole left behind for those truly diehard FFXI players who desire a new story in the world they love. A few minor tweaks here and there couldn't hurt, like maybe AoE threat generators and such to make tanking more intuitive. Party leveling is something that should come back, it was a great way to create connections in the game, and it's something that I miss about MMOs now. MMOs these days are all solo except for instances at certain levels until you hit end game, and it really hurts the communities.

I know the likelyhood that this will ever happen is about slim to no chance in hell, but I think it's still worth sharing as an idea.