That is simply impossible because Bitcoin was not designed that way. If we add privacy features into the existing protocol (eg. though some weird soft fork) that won't fix anything because opt-in privacy is not improving the whole system.
Why not? RingCT in Monero wasn't a thing back in 2016, and it got implemented in January 2017. XMR amounts hidden, boom. More privacy gained. Bitcoin could have optional privacy with softforking to confidential transactions and ring signatures. It's just that we want it to enter the system like a trojan horse, therefore we can't risk with stuff like that.
It won't solve anything for the same reason people are still using P2PKH/P2Sh addresses even though we added SegWit many years ago and it offers a lot of benefits.
An opt-in option is not going to help much with privacy specially in a world where centralized places are rejecting CoinJoin transactions (opt-in) they can reject the said opt-in privacy feature too hence forcing its adoption to remain low which means it won't solve anything.