The hell? You can level your bird past 10? Seems a tad pointless, but still kinda cool.
14 is all but confirmed to be a distant continuation of 12's world. They like to keep things open to interpretation, but unlike other crossovers, its built into mythology instead of being summoned from somewhere else.
You can reach the Return to Ivalice set of three 24 man raids, when your lvl 70 in Stormblood. It weaves FF tactics and 12 together, along with a brief mention of characters from Vagrant Story.
Then when you beat shadowbringers, you can start the Bozjan questlines and indirectly assist Dalmasca in two sets of Adventuring Forays. Basically open world battles with up to 70 or so players. Cool at first, but repetitive if your looking to grind for certain things. Costumes are not too hard to get thankfully, You can dress up as a judge and certain ff12 characters.
It is such a dam shame what is happening with Blizzard these days. Didn't Yoshi P want to do a collaboration event with em? That seems less and less likely these days.