There seems to be no benefit to doing lower tiers of dungeons when your character can easily handle higher tiers. Yet we are required to waste keys and time running lower tier dungeons to unlock upto Tier 4. This gets even more annoying considering multiple characters each having to unlock all 3 dungeons + Arena upto tier 4. Ideally the dungeon tier progress would be shared across all characters but this seems to be against the game’s design as even Monolith progress is not shared.
My suggestion would be to atleast make it so that progress is shared and tiers are unlocked across all 4 dungeons+arena. So characters don’t have to waste keys and time running lower level dungeons again. This way, you unlock Tier 4 in arena(most common keys) or Temporal Sanctum(most useful low lvl dungeon) for one character and this then unlocks T4 Arbor/Bastion too(only for that character) as you are clearly strong enough to do these.