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.
i don't know if it's the starting scores that really put MAD classes at a disadvantage so much as the progression that comes after that, i mean, it certainly doesn't help, but it's usually quite possible to get two decent scores and even something workable to stick in a third if needed especially if using array or point buy, but it's after that point, where two ASI only every four levels have to be assigned in different directions either slowing your progression to a crawl or going all in on one, and that's before you even start considering being able to take any feats!With point buy and background increases to ability scores, MAD classes are not a problem. My monk started with 16 wis/16 dex/14 con/10 str/10 char/8 int, which is great.