Option to add other services to your account individually as an add-on instead of having to switch plans. And more options to pick and choose.
For Example:
1) If I have Proton Pass Plus and want to add Mail Plus, have an option to add Mail Plus by paying $2 extra.
2) If I have Mail Plus and want to add an account for my wife, intead of paying more than double for the Duo and all that extra storage, just add it as an Add-on for, let's say $3.99 (mine) + $2 (my wife).

This way, if you're already on a discounted plant or lifetime plan, you can get prefferential pricing for already being a member without loosing your original plan and just adding on top of it.