Limiting troop tier and equipment by clan level could work.
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Lvl 0: 0 Troop Slots.
Lvl 1: 2 Troop Slots + Tier 1 Armor/Weapons - 50 Skill Cap
Lvl 2: 4 Troop Slots + Tier 2 Armor/Weapons - 75 Skill Cap
Lvl 3: 6 Troop Slots + Tier 3 Armor/Weapons - 100 Skill Cap
Lvl 4: 8 Troop Slots + Tier 4 Armor/Weapons - 150 Skill Cap
Lvl 5: 10 Troop Slots + Tier 5 Armor/Weapons- 200 Skill Cap
Lvl 6: 10 Troop slots + Tier 6 Armor/Weapons + Retainer, an expensive troop with 300 Skill Cap
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You could even unlock more stuff to make some tiers feel more special, like: Clan Tier 4 Unlocks Crafted Weapons for Custom Troops, potentially with the con of increasing wages by 25%.
Heck, it could allow for more interesting perks too:
Roguery 150, "Forgiveness by duty" - > Looters can be upgraded to your Custom Troops' Recruit equivalent.
Trade 150, "Arms Trader" -> Decreases custom troop wages by 10%.
I'd make a mod like this if I knew how and still cared enough about Bannerlord to warrant the effort.