With that in mind, based upon what you have seen with common multiclass combinations and such, what sort of classes or subclasses do you think are lacking in the game as-is?
It depends on what you mean.
I could answer that question with 3-4 different answers.
Do you want me to answer in regards to what character concepts can't be reached and effective by some specific level using the current multiclass rules?
Do you want me to answer in regards to what character concepts are mechanically ineffective at level 5?
Do you want me to answer based on what I can fluff around an existing class to get the concept I want?
The hybrid system is far from perfect in that regard, as it can be just as dependent on the subclass abilities themselves. Not all subclasses were created equal. It is too absolute of an argument, IMHO, to say that a hybrid allows for "the exact character concept in a mechanically powerful way." I would instead argue that it allows for an approximate character concept. It's often that dissonant approximation that creates the rub for many a multiclasser.
What makes you think anything I say is so absolute there are no exceptions ever? It's easy to beat down a viewpoint when you constantly characterize it in the most extreme way possible.
Multiclassing can give a sufficient final product at higher levels. I've been adamant that it's not as bad post level 5. It does have mechanical issues at level 5. Thus, the popularity of eldritch blast multiclass combinations. Eldritch blast bypasses the normal mechanical issues of 5e multiclassing and thus you see a large number of eldritch blast multiclasses. It's one of the most common examples.
However, a class/subclass designated explicitly to your concept should never have mechanical issues and will have an organic feel when it comes to leveling as your character concept of choice. The class/subclass may not exist yet but that doesn't mean that one couldn't be created that fit your concept nearly perfectly. There's always going to be some level of approximation seeing as we are dealing with a discrete leveling system that works on tradeoffs (take one thing instead of something else).
Now with the right class/subclass you should be easily able to hit post level 5 and have your concept in tact the whole time from level 1 on. Then multiclassing because useful to give some variability without having to create many slightly different iterations of similar classes/subclasses.
Herein may be one of the other points of contention in this discussion. If I understand you correctly, you place a greater value on what you perceive as reaching "powerful class abilities" earlier over what a character potentially gains from multiclassing into another class. This strikes me as a difference of priorities rather than an accurate assessment of how "piddy" mutliclassing or those other class abilities are. You seem to be advocating for "vertical class progression," for lack of knowing a better term, that values obtaining depth through gaining "more powerful" class abilities sooner. I think many multiclassers tend to place greater value on "horizontal class progression." They do not mind delaying powerful class abilities, because that is not their objective. Many generally prefer creating well-rounded characters, or, in other words, a character with "horizontal depth" rather than "vertical depth." This leads to the question of whether which approach has the greater volume. Obviously that depends on the particular build. But I don't think that a single-class character is inherently more effective than a multiclassed character.
How can anyone argue when you put it in that light, but that's not quite the case.
It's all relative to the other members of your party. If everyone decides to multiclass 2 levels pre level 5 then it's not nearly as big of an issue. The DM can scale encounters accordingly and all is well since the party is on roughly an even playing field. However, if the rest of the party takes the big jump at level 5 and gets their level 5 abilities (extra attack, fireball, spirit guardians, stunning strike, etc...) then you will be significantly behind until you get one of those abilities as well.
You can try to paint the picture that you are gaining more "options" and versatility but what you may gain in versatility in 5e is not going to come close to bridging that power gap. There's not enough horizontal versatility to be had pre level 5 to accomplish that goal.
As I stated above if the idea is a well rounded character or to fulfill a character concept then classes and subclasses will accomplish that goal better pre level 5. It's just the class with the features in it you want needs to first be designed.
