@mellored,
@Corinnguard
Regarding feat chains, I prefer to avoid them. On the other hand, a sequence of class-like feats that are gated by specific levels would be fine. Then one can ignore an unwanted feat, taking something else instead, and wait until higher levels to get the class-like feat that one wants.
Thus there can be a sequence, but there is no "chain" of one feat forcing an other feat as prerequisite.
The levels of a class actually are more like a "chain", because a feature at a higher level is often an upgrade of a feature at a lower level. The feature is too powerful for a single level, but as an accumulation of several levels worth feats it stays balanced. This is especially so for caster class levels, where casting a lower level spell is powerful, but doing it twice or thrice, plus spells at even higher slots would be too much for single level.
Therefore, "feats should be nice, not required" remains true. But a class level whose class feature is about the same amount of design space as a feat, represents a "chained" commitment to a series feats for the entire career of the character. One can multiclass, but then one is starting over at the low-level feats of a class.
A subclass is a significant design space. In 5e so far, it functions as an auxiliary class, mainly for the purpose of specializing. But it too is a "feat chain" commitment like a class is. But this design space, when standardizing can be used for many different kinds of commitments. One might use it for subclassing in a way that doesnt lose levels in the primary class. One might use it for "prestige class" concepts, that only require four or more levels to express the concept, and be a concept that many classes can benefit from. Also, it should be possible to "multi-subclass", switching back and forth between more than one subclass as one advances up the levels.
The subclass is already an extremely important design space. Yet it only opens up its creative utility when its levels standardize across the classes.