Probably the answer will differ per person here on what they would like to see, this is what I think would be important.
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Testnet exchange should be compatible with the workflow of real exchanges, e.g. by turning on the option mainnet, they should be able to use the code with minimal modification to implement Grin on their own exchange.
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Should work as a minimal online wallet showing Grin saldo of the user. Users should be able to test receiving and sending coins from testnet to their own wallet.
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Should work with slatepacks, e.g. showing the slatepack when logging in on the online exchange account. This is important since an easy to implement testnet exchange should empower real exchanges to implement slatepacks
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Preferably this project should involve some exchanges since there is no other way to know what exactly is their current problem with implementing slatepacks. Pobably the problem is that reaction can bounce since they are interactive, meaning they need some queing mechanism, would need late locking and.
Probably others can better formulate a requirements list, but this is what comes to my mind.