A little bit tangential to the direction this thread went off in, but regarding some of the comments at the top of the thread, I think one of the angles to consider is the user accessibility and game design sides. If we're discussing significant rules changes, I'm assuming a theoretical 6E or 5.5E because you probably can't maintain backwards compatibility with this kind of fundamental change.
Whatever it ends up being, we know that one of the ideas on the table is removing bonus actions. It's not that bonus actions are particularly bad in theory, most of them are fine, but they have caused the game designers no end of headache and have caused a lot of rules confusion among players (just have a look through Crawford's twitter feed to see the number of questions about bonus actions, and the mental gymnastics he goes through to explain what actions are allowed and in what order - and yes, sometimes the answers change with time).
Similarly, we know from various rounds of Unearthed Arcana that one of the major hurdles they face in designing new classes/subclasses is pinning down the multiclass behavior. (There are also no end of questions online about multiclassing - particularly regarding spells and spell slots). A significant issue is that in 5E, there is explicitly no attempt to balance classes at any given level (rather, the stated goal is to balance them over the array of levels). Classes have totally different growth rates, even within the same class (for example, various Wizards subclasses: the Evocation wizard gets their best ability at level 2, the Transmutation wizard gets their worst ability at level 2).
So, whether or not anyone likes multiclassing, we know that in its current iteration, multiclassing causes the game designers problems and we know that it is a source of confusion for players. In this light, and given the explicit goal of "appealing to the masses" it would not particularly surprise me if the game designers left it out of the next iteration. To me, feats seem like a decent alternative worth testing out because they are much simpler to tack on to a character than requiring detailed knowledge of multiple classes on top of special rules on how to combine them (in my experience, a lot of people don't fully understand their own class even on a single class build).