My hope is that, based on the timing and when the expansions for FH3 were unveiled and released, Playground both unveil and release the second expansion on the same day during the Microsoft E3 conference. It's a 99% bet that we, at minimum, get a trailer or gameplay demo.
As for the setting, if that rumour is true, I might say Minecraft - but I don't know what they'd do with that setting, that wouldn't require an almost ridiculous amount of new asset work for a mere expansion.
Microsoft's E3 conference is probably gonna be bananas this year. New console(s), new next-gen Halo gameplay (probably), possible new Fable unveil, Horizon expansion, news on whatever some of their new studios are making, Project xCloud, and who knows what else. Plus they are the only game in town this year for third party reveals, so they'll have buckets of those probably too.
And new Battletoads!