In any case there will be 2 spin off (for now we hope 2-3 more later)
1 which will be really the continuation of the manga, so the logic wants that is the quest of 100 years, the key of aquarius, the new generation.
These are the 3 main ideas for a logical manga suite.
So if it's a continuation of the manga, I do not see it spoken of CS, laxus, sprigans or others, will be on the Natsu team.
After he can talk about it in the spin off to give info on that, but not make a spin off that on that.
That's why there will be another spin off after these 2, in the manner of Lost canvas with these spin off "" CHRONICLE "", that I would talk about CS, sprigan and all.
But in this spin off which is the continuation of the manga, he can do the 3 (the quest of 100 years, the key of aquarius, and the new gen).
Simply the story begins, with the natsu team, who talks about the mission but also about CS what they have become.
Also talk about sprigan and alvarez.
He arrives at the place of the mission, begins it, does it with difficulty and everything.
Succeed but get stung the object of the quest or someone is kidnapped by a thug / thief.
So the Natsu team run after them and leave the country to go to Bosco as an example.
He searches, searches, finds the person or object and returns home to the guild.
And at the same time during this visit to Bosco lucy finds the key aquarius.
Well it says very simple but you see the idea is a simple scenario, effective and that will please, we ask no better in short.
And the end of the spin off in bonus chapter, 2-3 chapter on kids, voila.
Because of course mashima having read and received com people, will probably nalu and jerza in the spin off.
For jellal to make her marriage proposal to erza, what the fans want.
And for the nalu an even closer reconciliation, see a declaration of love made by lucy, stuffed or not XD.
But that will not remain without consequences XD.
Because it will not come from natsu, or so in its way XD.
People want this THE LAST FT version, everyone shouts and claims.
By the way, did you read the story, which I showed you?
But in short, it will be "" original "" if this statement came from lucy, history changed that I am in 99% of cases the guy who declares himself first.
As for the other spin off, bha must see what he will do in the first, according to what he does it will determine what will be in the 2nd.