Wait a minute, third act of Infinite Frontier, Dark Army, supposed death.... I think I've figured out where the League went already and what's happened to them. It's Pariah.
Okay so some context for those who haven't been reading, at the end of Infinite Frontier Barry goes beyond the known Multiverse and runs into Pariah and the dead remains of Multiverse-2 which died in the first Crisis but has been fed upon by the Gentry/The Empty Hand. But then Pariah disintegrated Barry and remade him on a picturesque world designated Earth.Flash.1.
That's where the league will be, captured by Pariah and put in their own perfect earths but made to look like they died. I'm confident in this.