This is just a what if. There is no sudclasses

[Dr. Cliché]

I think this is an interesting idea, albeit a little flawed (though not fatally).

So, let's say I was playing a Fighter. When I get to Lv3, instead of choosing a subclass, I'd instead choose one of the following abilities:
- Improved Critical
- Combat Superiority
- Spellcasting & Weapon Bond

Then, when I reach Lv7, I can choose between one of the following 3 abilities:
- Remarkable Athlete
- Know your Enemy
- War Magic

And so on.

(I haven't included the options from UA and supplements, but hopefully you get the idea.)

See, I actually think that this could be a pretty good way to build characters that, for whatever reason, don't really fit into one of the established subclasses. Or just messing around and creating your own subclasses.

Anyway, I see four main issues with this:

1) Many subclasses do not have equally useful abilities at all levels. There is no getting around this, though I don't know whether it would create any serious issues.

2) Some abilities are taken at a specific level but then continue to scale afterwards. The obvious example here is spells - an Eldritch Knight gains spellcasting at level 3, but then his spellcasting ability continues to advance all the way up to Lv20 with no further investment in abilities.

The only solution I can think of here would be to say that if a character takes a Spellcasting ability (e.g. from Eldritch Knight or Arcane Trickster) and then subsequently takes an ability from a different, then they lose any further spell progression.

3) Some abilities are reliant on previous abilities. For example, the Sorcerer's Elemental Affinity ability is based on them having chosen a dragon type with their Lv1 ability (which wouldn't be possible if they had instead selected Wild Magic abilities at Lv1).

This, I think, could be solved by simply closing off any abilities that are reliant on previous abilities. So a Sorcerer who took Wild Magic at Lv1 wouldn't be able to select Elemental Affinity at Lv6. However, they would be able to select the Lv14 and 18 abilities as they aren't technically reliant on the sorcerer having made a specific choice with an earlier ability.

4) The fluff might not work. In the example above, the Wild Sorcerer would be able to take the ability that allows them to manifest Dragon Wings, even though their powers don't come from Dragon ancestry.

Personally, I think this could be refluffed pretty easily. A Wild sorcerer probably manifests all sorts of weird stuff. A Shadow Sorcerer could use wings seemingly made of darkness, that sort of thing.

So, in summary, I really like the idea but I think it would need some fine-tuning to make it practical.