How do you think class progression should be set up to be better than multiclassing?

[Rynjin]

You design classes the way Pathfinder 1e did them, especially taking into account the lessons they learned toward the end of the game's active development.

D&D 3.5 had the exact same issue; multiclassing was pretty much mandatory for most characters, either through rampant dipping because so many class had frequent dead levels, or through Prestige Classes for casters in particular, because it was the only way to get real class features that weren't casting, without giving up your casting progression.

Paizo didn't like this paradigm, so they did a few things with the express purpose of reducing the need and desire for multiclassing,a nd to make level 1-20 in a class the norm:

1.) Favored Class Bonuses. For every level in your favored class (chosen at first level), you get a small bonus; +1 HP, +1 skill point, or a similar minor buff based on your race that could be applied to certain classes.

2.) Every class was designed to be a complete package. In the early days the effectiveness of this varied, with classes like Fighter in particular still being dip-fodder, but every class was MEANT to feel like it became more...itself as tie went on. Much later, with Advanced Armor and Weapon Training options, and Weapon Mastery feats (which technically anyone could take, but had to jump through hoops to get) even Fighter got this. A Level 5 Fighter should feel more...Fighter-y than a level 1 Fighter, or a level 5 Barbarian.

3.) Give most classes an ability at EVERY level. This gives you a psychological "hook" to stay in the class. "Well next level I get, so I can't swap, and then NEXT level I get..."
--3a.) Talents. Most classes in the game get some kind of talent every 2 levels (some starting at 1st and repeating on odd levels, and some starting at 2nd and repeating at even levels): Rogue Talents, Alchemist Discoveries, Witch Hexes, etc. This not only gives you an ability at every level, or close to it, it always gives you a CHOICE to look forward to, not just a static ability you have no control over. Minor or major boosts you CHOOSE to customize your character further.

4.) Archetypes. Give players the ability to swap out class features they don't like, for ones they do. These in many ways take the place of multiclassing specifically, to the point that many common (and even uncommon) class combinations become redundant. Do you want to be an Inquisitor of the god of nature? Well, then you want some more nature-y abilities, yes? Why not be a Sacred Huntmaster, and trade out a few of your abilities for a Druid's Animal Companion and a couple of other goodies? And so on.
--4a.) More classes, particularly Hybrid classes. It was long said on the boards that a hypothetical gestalt of Rogue and Fighter would actually be a balanced class, because both classes are so insanely weak on their own. Paizo pretty much put their money where their mouth is on this one, with the Slayer: a full BaB class with a ton of skills, Sneak Attack (but lower progression than Rogue), Weapon Training style bonuses (available from 1st level, but limited to one target at a time and taking an action to activate), and access to both Bonus Feats and Rogue Talents via their Slayer talents. This actually killed two birds with one stone, as it also acted as the "spell-less ranger" everyone wanted. It is an extraordinary class, and very well balanced.

5.) A reduction in Prestige Classes. They exist, but over time mostly became obsoleted by classes and archetypes; why be an Arcane Trickster (a Wizard/Rogue PrC) when you could be an Eldritch Scoundrel Rogue (a Rogue that trades some Sneak Attack progression for spells) and not have to plan your build exhaustingly from level 1 and struggle through some very clunky levels?

6.) EVERY class has something level dependent that scales. Multiclassing is an actual sacrifice for most characters, not just casters, because you're missing out on something cool you have that could become even cooler later.

What all these things add up to is a game where multiclassing still exists, and can be powerful if you know what you're doing, but is entirely optional. You can, and are encouraged to, pick a class that appeals to you and stick with it because it allows you to play the exact character archetype you imagine in your head with minimal effort; everything is done at charop and from there natural progression takes you forward. I've made well over a hundred Pathfinder characters and the last time I remember multiclassing with one was...2014, maybe?

5e does not take most of these lessons to heart. A lot of your major character choices are chosen at levels 1 and 3, when you pick your initial class/race, and then get your Specialization. A lot of these things don't scale.

Unless you're an Eldritch Knight, what incentive at all do you have to stay Fighter in Core 5e? You pretty much have all your class features at 3rd, and then get your Extra Attack at 5th. If you really like your level 7 ability, maybe tough it til then. Champion gets some extra +numbers abilities, but you could get similar +numbers stuff from other classes, and probably have more fun doing it. Battlemaster gets a few more Maneuvers known, but if you're happy with the three you have then there's little incentive to stay. After all, your Maneuvers continue to scale at the same pace since the DC is based off your Proficiency mod (shared across all characters) and your stats, instead of 10+1/2 your level+Stat.

You gonna wait until level 10 for an average +1 on your Maneuver dice? Or would you rather take 3 levels of another class instead and get an entirely new Specialization ability at 3rd, plus other stuff like Fighting Styles, etc.

These lessons have already been learned, and I'm sure could be adapted and simplified to work in a hypothetical 6e which still wants to take a rules-medium approach.