While xrealm dungeons aren’t super likely, xrealm BGs are already confirmed to be in as of the FAQ released today specifically saying that they are in phase 3.
Q: Will there be Cross Realm Battlegrounds when those go live?
A: Cross-realm battlegrounds were a feature of 1.12 and are important for matchmaking, so you’ll be seeing cross-realm battlegrounds in WoW Classic when Battlegrounds are introduced in Phase 3 of the content roll-out plan.