Has there been an attempt at adding LTC or any other cryptocurrencies to BTC or another altcoin? Specifically I intend to trade crypto-crypto without an exchange.

I propose a simple scheme, all nodes sync the blockheaders similar cryptocurrencies, taking up a trivial amount of space, and a new opcode is added to BTC to verify that a specific block exists within a specific blockchain.
This would allow for the following transaction to take place:

Suppose Alice wants to trade 10 BTC for Bob's 10 LTC:

• Bob crafts an LTC transaction to send 10 LTC to Alice's LTC wallet.
• Bob does not broadcast this to the chain, but sends an unsigned version to Alce
• Alice creates a BTC tx to allow Bob to spend 10 BTC on condition that Bob's transaction to Alice exists on the LTC blockchain
• Alice publishes her transaction to the BTC blockchain
• Bob can spend the transaction that Alice sent if he can prove that a specific block on the LTC chain contains the Tx that he sent Alice
• Therefore, Bob must publish his transaction to the LTC blockchain

Obviously, one would need a timeout condition, e.g. allow Alice to spend the BTC if it has been over a day, otherwise Bob could simply burn Alice's BTC by never sending Alice 10 LTC on the LTC network.

Any thoughts or insights?