Exalted Forge: Found deep in the Arena, the Exalted Forge is a limited-use station that allows a player special crafting opportunities. Instead of using your own shards, glyphs and runes, the Exalted Forge is stocked with a small number of special items that are randomly selected.
Special, powerful, but limited and hidden behind the grind of the Arena, these would be valuable crafting opportunities to motivate players into the Arena beyond Experience Grinding and Ladder Competition.
Crafts could include:
Upgrade a Mod to Exalted / Upgrade Existing Exalted Mod
Rerolling a base Item
Rerolling values on an Idol
Rune that has a small chance of rolling Unique (Limited to Gamble-able?)
Perhaps Arena-Only Affix Shards, or selection of rare/class-only Shards.
Whatever you leave on the Exalted Forge is gone when you leave. Perhaps you are not only limited by the choices, but the forge runs out of power after a certain number of uses.
The Exalted Forge could be similar to a Quest Echo in Monoliths, where you need a minimum number of Arena floors to see your first, but then your odds are just improving for the next one as you keep going higher. Maybe guaranteed every 80-100 floors, adding additional use to that Golden Key.
Still leaves Monolith as the main way to farm Bases and get initial items, but now Arena has a unique niche it can offer that supplements that loot instead of directly competing.