Will the wallet app itself start implementing some warning/alert option to inform the receiver that the coins are "tainted"?
If a UTXO is banned from coinjoin participation (whether it is due to DoS protection or from blacklisting), the user is showed a message that says "Some Funds Are Rejected From Coinjoining" and a
/!\ symbol is displayed next to the UTXO in the coin list which can be hovered over to show the unban time.
That's where it starts in my opinion, and it will open a can of worms. Because what if the most popular wallets also started implementing warnings and alerts that the inputs coming in their wallets are "possibly tainted"? How would that make the users feel and what would they do? Those coins would be sold right away, and if enough users are "warned/alerted" by their wallet app, it's probable that "tainted" Bitcoins would be valued less than "clean" Bitcoins.
I believe Lightning's real value propostion is in user privacy and fungibilty, not cheap and fast transactions. Isn't there an update that makes the opening of channels look like regular transactions?