Concept of other worlds

[Enkidoh]

If I remember rightly, Yoshi already revealed that it would be the Crystal Tower that would be the gateway between the Source and the Shards (hence why a copy of the Crystal Tower appears in the First and is featured front and centre in the full trailer - it's Yoshi pointing his thumb to it and saying "THIS PLACE IS GOING TO BE IMPORTANT FOR THE NEW EXPANSION" (subtlety is hardly SE's strong suit).

As far as Evil!Derplander and his party are concerned, it's clear now that Elidibus merely lied to them that death was theonly way to travel to the Source (remember: Ascians lie), as it not only served his ends it also allowed a convenient means of eliminating them if they decided to slip their Ascian-held leashes (sending their souls onto the Lifestream), and that physical travel between the Source Hydaelyn and the Shards is possible, although it could just be a handwave to allow Shadowbringers's premise to even work. Either way, we'll find out soon.