If I was building a particular game that did not include subclasses for any of the classes and replacing their slots with ASIs/feats... what I'd do first would be to take all of the subclass abilities and work/rework them and turn them into feats themselves. In my opinion the number of feats available in official sources are too limited to the amount of additional ones going to be picked up.
If every class is going to gain three to six additional feat slots, we're going to see the exact same feats taken by more than one character in any particular party, which to my money runs counter to the purpose of feats in the first place. Feats are supposed to give PCs a little definition and originality... which is easily foiled if half the party is "Lucky" for example.
As far as I'm concerned... any mechanical feature a PC has-- whether it be a class feature, subclass feature, racial feature, background feature, feat, magic item etc.-- can be worked/combined/separated to create relatively balanced "packets" of game mechanic ability. Then players can choose those packets they want to create their slate of game mechanics for their PC. There's no necessity to just sticking with the various features the Player's Handbook has parceled out in the format they have. Sure, it's incredibly easier to do so... and these pre-built packages of features have been balanced and storyboarded to make them much simpler to grok from both a mechanical and a story point of view... but anyone with a little bit of time can reverse engineer all this stuff to create all kinds of different things if they feel like they want to and they have players that would appreciate it.
You just have to be very comfortable with whatever it is you end up creating, with the expectation that it hasn't been playtested at all before you hand it to your players. But if you're good with working on that stuff on the fly... then go nuts.
You may want to check out Pugmire. It has exactly that mechanic, all class abilities are feats, feat every level.