Large capacity tanning building, similar to windmill, would be nice and would help cut down on the number of tanning tubs stacked up by high capacity guilds. As well, it would cut down on server lag since it would reduce the particle generations to just a few tanning tubs.
I see 5 total tiers tanning:
Tier 1 - Tanning Tub (1 item/use) Carpentry
Tier 2 - Tanning tank (5 items/use) Construction
Tier 3 - Small Tannery (20 items/use) Construction
Tier 4 - Tannery (100 items/use) Masonry
Tier 5 - Tanner's Workshop (500 items/use) Architecture
The higher capacity tanning builds could even become more of a resource dump for a guild, requiring charcoal, water, and perhaps even negative preparations to fuel them.