@ Eratas: You could do something like Poyo Rainyday's artifact. Have Tabane (or Phantom Task for that matter) inject the protagonists into a virtual reality (matrix-style
).
That world would be perfect. It's each of the protagonists perfect existance. The world would be at peace, no wars, the IS could or could not exist, etc.
Ichika could have his parents. He'd be living a simple life. He'd have Houki and Dan and Rin, a really simple life, the life he had before ISA. What he views as his perfect world, without murderous yanderes or attention he doesn't like.
Kyuukai could have his family. He'd be innocent, unscarred, mentally stable. The wars never happened, he became Tabane's and Ryuu's blood-related child, Koyomi would have her father and he wouldn't be dead. Caitlin would be alive, his life would never roll downhill. And his life becomes an escalator, easily taking him up to where he wants to go.
I don't know what Irish would like though.
Tabane could use this reality as a test. To see if the protagonists want to leave or not, to mentally condition them, prepare them. She's perfectly content with leaving her son in there for the rest of his life if she had to... Visiting him with Koyomi by her side every day.
But if Phantom Task puts them in, it could be a tool to milk out information or subdue the protagonists.