Yep, that's the rub. Having all class features available at 1st level (even if they are nerfed or hobbled) isn't really compatible with multiclassing. But that isn't an argument against 1st level characters getting more features--it's an argument against the current multiclassing rules.
Which I actually like. I could see different ways to do it though. Probably if you can chose a multiclass subclass right at level 1. And at some later point you can chose a 2nd and maybe even a 3rd subclass...
A bit like 4e' paragon path and epic destiny. And later the option of dual classing.
The only alteration I'd do would be that there are no special paragon classes or epic destinies.
Instead you can always either:
- pick up a 2nd/3rd/4th class.
- improve your 2nd/3rd class.
- take a specialization.
An example.
You start as a fighter level 1.
You can now chose subclass wizard or become a battlemaster fighter.
Lets assume you became a fighter/wizard. Now you can chose to become a battlemaster now, become illusionist. Or improve the wizard subclass. Or even take a 3rd class.
At level 13 you get to improve wizard or take another subclass of either wizard or fighter.
The features you get from subclass or multiclass spread over 6 levels.
The wizard would grant two spellcasting levels. So you end up at 6th level spells at level 17.
Or something like that. Maybe features of the first subclass you take spread over the whole 20 levels. The advantage of that approach would be multiclassing or subclassing right from the start. You improve your main class features over 20 levels. You can chose to go deep or wide: Specialization or multiclassing at different points.
Disadvantage: its less flexible because there are certain decision points.
That could actually be fixed by allowing to chose when you get some abilities.
In a perfect system, you might have heroic features, paragon features and heroic features.
At every level while you are heroic you can chose any of the 5 heroic features. If you got all of them you become paragon and now can chose between them and so on.
A bit like pathfinder 2e class feats, but less features overall. Eventually you get all of them, you just chose the order you want to get it. And way less fiddly.