Lightning Splicing. (Still in the works.) You can "splice-in" to add funds to a LN channel on the fly, with no downtime, in 1 transaction. (And it looks like a normal on-chain payment with taproot.) Also, you can "splice-out" and remove funds (bring them on-chain) with 1 transaction.

Whats more, these splice-out transactions can be batched to do a coinjoinlike transaction that is CHEAPER than a normal transaction.

You can learn about it in this talk by Dusty from Adopting Bitcoin Conference in El Salvador: