Limbo

Limbo is a dimension independent from all timelines, which characters capable of timeline travel temporarily occupy whilst traveling. They may also remain there indefinitely, or until they wish to leave.

Appearance
Limbo can be thought of as a near-void. No physical objects or matter exist here; all visual phenomena are conceptual. The space occupied by travelers functions as a hub world hosting portals to all timelines. Portals can be marked to indicate previously visited timelines, failed timelines to avoid returning to, among other things.