This one: https://bitcoinops.org/en/topics/eltoo/

Lightning network allows fast transfers and helps privacy, but the nodes are difficult to run due to the need to store the whole history of channel states (to publish a penalty if your peer cheats). The database size grows constantly and this limits how many transactions a node can route for the lifetime of a channel. There is also a need to have a watchtower - a third node to monitor a channel in case my node goes offline for a long time. All these complications can go away if a soft fork would allow eltoo. Not a hard fork even, but still unlikely to implement.