Here's my idea:
This could be a T6 room and there should be one gate room per lair only, the Helian one should provide a Helian gate and the Lunus room a Lunus gate (obviously).
This gate should be usable by the lair owner only and could lead to the main cities, Bristugo included, for free.
To build this room, a permit could be required (like vault, pawnbroker, consigner and tavern), in addition to special component(s):
a new quest line could be created, available at lvl 100 Lairshaper, given by the Lairshaper trainer who could send the player to Nielenoss the Dragon Tinkerer, as building a portal is a tinkerer work.
Any suggestion that can complement this idea is welcome.