There is a demand for that, but as harizen mentioned, you'll face trust issues. With the recent hacks on various exchanges, exit scams from ICOs, even abandoned exchange/DEX it is quite difficult for most people to blindly trust any new exchange even though they promised to provide instant liquidity. You might be able to do this if you can provide a smart contract. It should be able to receive/detects transactions from various blockchain and in turn send back what the sender should receive, or in other words, atomic swap.

Other solution would be to make a multisig address where you and someone trusted operate the exchanges, but this won't provide instant liquidity as it requires you to sign manually afaik. On top of that, you should address regulatory issues too. If you have high liquidity, there is a possibility that government will 'track' you down and requires you to follow the regulations for exchanges, unless you live in a no man's land.