Not exactly on topic, but definitely topic-adjacent, I find it extremely frustrating that all existing wizards get a power boost with being able to cast 18 of the 21 new spells, with the other classes essentially getting scraps.
I mean, was anyone complaining that WIZARDS were underpowered?
Some people were, but every class has people claiming they're underpowered.
Regardless, it all has to do with the mechanics of the classes. Each class has two lists: Spells available and spells prepared. Their limitations for these lists are the reasons why we see disproportionate numbers of new spells in some clases.
Clerics and druids, generally, have all spells available, and can prepare a large number of them due to bonus spells they get to prepare (cleric more so than druid). They have the widest availability of magical options and leave less options unexplored. Adding a spell to their spell list is a notable boost in power due to the smaller lists they have.
Wizards can add any spells to their spellbook and get two more every level. They have fewer they can have prepared, but still are the most versatile of the arcane classes. They have such a wide range of spells that any new editions are minimal impacts (unless overpowered).
Bards and sorcerers have shorter lists of spells they can learn, and their available spells are also their prepared spells. This is usually a huge restriction, although some subclasses (aberrant mind) open the floodgates by having a ridiculous number of spells prepared (on par with the cleric). Adding spells to their lists is also not that big of a deal, because - generally speaking - their 'choke point' on magic is the small percentage of the spell list they can have prepared, and how hard it is to change it up.
Because of the bard and sorcerer limitations, I feel like they could open the flood gates on these classes. However, they try to keep it thematic.
Warlocks are the real odd ball class as they get to know so many more spells than they can cast. This encourages them to build a very different spell list. They're best off picking a couple heavy hitter spells that they'll use all the time - but after that, they can select really conditional spells that they'll only need once in a blue moon. It results in them sometimes having that one odd spell that is only useful once in a blue moon - but when you get to use it in the right situation, it is devastating.
I hope that spells is something they improve for 6E. If I were rebuilding spells, I'd do it by breaking magic down into two categories: Auras and Spells. Each class would be able to learn a certain amount of magic from a list of magic, with each class having their own breakdown of how it works, but essentially each would have a specified amount of magic known, a certain amount of magic prepared, and a certain number of spell slots (just as we do in 5e).
If you had learned an aura and prepared it, you'd lose a specified spell slot and gain an ability until the aura was no longer prepared. For example, if you had the Detect Magic Aura prepared, you would lose a 1st level slot, but gain the benefits of the 5E detect magic spell at all times. Every class would have different restrictions on how many auras you could have, with Warlocks being limited only by their number of slots, and classes like ranger and paladin only getting one, ever. Many ritual spells in 5E would become auras in my 6E. I think this model would simplify a number of things, allow spellcasters to effectively concentrate on multiple spells without it being overpowered and really up the differentiation between classes and subtypes by giving them unique auras.
