The use case I have in mind is to connect to heterogeneous blockchains: BitCoin, and Ethereum/RSK:
The main problem is that both blockchains are too different. The resolution of both use case (transfer from BitCoin to Ethereum/RSK) will be very different to the resolution of the second use case (transfer from Ethereum/RSK to BitCoin).
And the key difference is that Ethereum/RSK has smart contracts (even native precompiled contracts, if needed). The second difference is the format of transactions: BitCoin uses Unspent Outputs, meanwhile Ethereum relies on keeping account balances.
But it is an interesting and non-trivial use case, to discuss in detail in the next posts.