Barbican being an NPCless, lawless place is really cool. I know there's a tendency for nostalgia and tradition when it comes to the distance between Upper & Lower, but the In Between itself has changed drastically. Other areas have been added, removed entirely, or moved across the Underdark. I think the health of the PvP conflict in EFU would increase if the Lower transition wasn't literally a single round of running away from the Sanctuary transition.
I'd be a strong advocate for the transition being moved further across the Inbetween to create more distance between Sanctuary and Lower. Alternatively, having an ooze-ravaged alley area positioned between the NPC safety of Lower and the Inbetween would be great as well.
During battle in the In Between with explosive spells & items, the archway between these two locations is actually quite often destroyed.
Inconvenient as it is for Upper folk, blocking this passage with rubble would certainly do the trick, and if it annoys too many adventurers it may encourage more people setting up bases in the wilds.