So currently, the maximum level of our barracks, both elite and normal are level 6.
In addition we have a large lack of content at a certain level (>56) and certain resource values of the elite kind (*cough* platinum ore) are worth less than water.
Proposal:
Offer 2 very long chains of quests to further upgrade each barracks separately.
The chain to upgrade the elite barracks could involve making numerous buffs which cost a LOT of platinum ore and various other resources. Tasks involving various AB adventures etc.
For example, the quest chain to a level 7 barracks could be:
1. Create 10 barracks modules at 50k platinum ore, 1k maho wood, 1k granite each.
2. run 25 woodcutters
3. <insert some random other tasks
...
x. profit (instantly upgrade barracks to next level on completion)
The follow-up could require say 20 modules, and running 20 first-thieves.
The 3rd 30 modules and running 40 2nd-thieves (hey it has lootspots ;p)
etc..
In theory such a chain could go on indefinitely or to say level 20, with each level decreasing production time of troops, but also significantly increasing in costs and quest chain length.
This would give even long-time players a goal to work towards. It would make certain over-abundant elite resources valuable or a lot more scarce even with the current adventure income. While slowly reducing the very very long production time of elite barracks units.
So, to reiterate: use quest chains that use currently overabundant resources in excessive degrees to make them more valuable/wanted as a resource sink, and give players some QoL/bottleneck-widening in return.