I like DEX of Trust Wallet would can change other currencies like litecoin, dogecoin, Dash, Bitcoin, Tron, and more.
Also BEP2 also tokens as Bitcoin PEB2 i like you can change for their native currencies in the DEX and that Stablecoins and other tokens and coins can be exchanged for any other currency.
I understand your idea, but technologically it isnt possible for today.
Dex in todays world is trade in blockchain. In one, certain blockchain. There is mo way to change btc to eth except exchanges. If world firsts crypto still dont have it, well, its not because devs are lazy, agree?)