If it is just rarity of the classes you want you don't have to base that rarity on the roll of stats. You can use point buy and if the player wants to play a paladin they can roll a d20 and if they roll a 20, they can play a paladin. The same can be done for other classes. Done.The chooseable options are the basics; if you want something special you have to roll for it and may or may not get it...this time. I do it this way for various elements of character creation, not just class. This allows rare-in-theory to be and remain rare-in-practice.
A lot of those ideas make more sense when you’re playing a high lethality game with a lot of PC turnover, AND you expect that the game will continue for quite a few real-life years.This tangentially raises another point: with point buy or standard array something I like to do becomes impossible: that being to gate certain classes etc. behind high rolls in order to make them less commonly seen in play and a little bit special if-when they do arise.