Okay, to clarify, i don’t think the concept behind multiclassing is bad and should be removed, being able to combine class archetypes and features, BUT i think the current incarnation of multiclassing is pretty terrible, classes have to be designed in consideration of the combinatorial potential of each other, a class you multiclass into IMO should not provide the same progression as if it were your primary class.
My preferred implementation would be everyone is technically monoclass, but with a very high degree of options of customisation that still lets you pick up other classes features, just in a way that doesn’t have to affect how the baseclasses are designed.
I like the multiclass rules as they are now.
But my preferred multiclass option is how final fantasy XI classes work:
You have one primary class and one secondary class.
Secondary class is always half the primary class level and a select few abilities are not available as subclass.
For D&D, I'd make it:
Level 1:
Option 1:
chose species, background and main class and a feat.
Option 2:
Instead of chosing a main class, chose two secondary classes. You get reduced features (like level 1 as muticlass as it is now, you must chose wither pact magic or spellcasting trait)
Level 2:
Option 1:
Chose a secondary class (a class that is half your total character level)
Option 2:
Chose a subclass (as they are now but improved to be as valuable as a class of half your level)
So combining this two options at both levels you either have:
Main class + subclass
Main class + secondary class
Two secondary classes + one subclass
Three secondary classes.
Resoning behind it:
Subclasses can be designed in a way that they are never combined with other subclasses. They are highly specialized.
So no eldritch knight + blade singer combination.
But you get a lot of freedom from being able to substitute a subclass for a secondary or even have 3 secondary classes without a specialization.
I am not sure how I'd design feat and subclass progression.
Probably just using total character level seems like a good idea. Another question is how spell levels add up,since it is possible to have a spell progression that is up to 1.5 spell levels.
Maybe spellcasters only add 2/3 caster level towards total spell progression as main class and 1/3 as subclass. And most caster specializations add 1/3 caster level progression. And you round fractions up (like eldritch knight and arcane trickster). Math is a bit ugly.
But spellcaster table could also be adjusted to have 30 levels instead of 20. And you get another spell level every 3 levels.