The whole point of a class/subclass system is to provide a container for similar mechanical and thematic elements.
Well, thematic yes, mechanical not necessarily.
Of course
completely different mechanics may look odd as alternatives inside a class, but also may not.
At least we already have examples about subclasses where the mechanics are the same but the relative weight of 2 mechanics is different between subclasses: the two druidic circles, they both share spellcasting and wildshape mechanics, one is slanted towards the first and one towards the second.
We also have an example of subclasses (Wizard) where a certain mechanic (rituals) is significantly different in a specific subclass (Scholarly Wizard): rituals can be cast from any known spell rather than only from prepared spells. This may or may not be a "different" mechanic for rituals, depends how you see it, but it certainly has potentially huge tactical differences...
Overall I certainly don't think we
need even more mechanics for subclasses, it's probably best to just limit the number of mechanics in the game, and focus the design work on making them as solid as possible. But the subclasses can
alter those mechanics, and technically can also introduce more, there's no fundamental limit to this.
For example,
Rage could have been a mechanic unique to a Fighter subclass called Barbarian. It didn't happen, because WotC designers have decided that Barbarian is a character archetype with enough breadth to deserve its own class and they chose Rage as a signature feature of such archetype. However, the Barbarian class is the only class that so far has never seen any subclass option, and maybe the reason could be that the archetype is a bit too narrow to create variations, but another reason could be that it is the Rage feature itself which is an encumbering presence in the class with a rigid mechanics: maybe we'll get barbarian subclasses with altered Rage mechanics or focus on something else.
A similar thing could have happened to
Ki, to be bundled in a Fighter subclass. However the concept of Ki is much more supernatural than Rage, and much more generic, so it should be easier to create lots of Monk subclasses by just designing tons of Ki effects.