My request was to be able to select a replacement spell if you are already granted darkness from some other game feature (subclass, race, whatever).
This might seem good at least as a clarification, but ultimately it is not needed and can cause unwanted consequences.
Classes with scarce numbers of known spells (Sorcerer, Warlock, Ranger and Bard, as well as EK/AT subclasses) can
already replace a known spell when levelling up, so they have no penalty for choosing these feats as-written even if they already know the granted spell.
Druids and Paladins don't already know Misty Step nor Darkness, so they
don't need a replacement.
Clerics may or may not know Misty Step nor Darkness (depending on domain), but they already know all their possible spells, so they
cannot replace it with something else from their class spells list.
Wizard already know a lot of spells, they don't really need a replacement, but then they are the
least likely class to take this feat, because they have much better ways to obtain new known spells (although variant humans might like the perk of getting these 2nd-level spells at 1st level, but what replacement do you need when you're at 1st level?).
If the character already has Misty Step or Darkness from a clerical domain, race, or special class ability (the only other case I have in mind is the Oathbreaker extra known spells) then
they don't need the feat in the first place, because Misty Step is
the primary reason for someone to take Fey Touched, and Darkness is
the primary reason to take Shadow Touched, not the 1st-level spell (if you are more interested in the 1st-level spell, take
Magical Adept which doesn't limit the school of magic and gives you also two cantrips).
So allowing a replacement will only create a possible confusion when a Cleric who already has it tries to convince the DM that they want the feat but deserve a replacement spell, and then try to get
any 2nd-level spell
from another class, which is NOT the intent of the feat. If that was the intent, then why don't they just do a generic feat that explicitly grants any one 2nd-level spell of choice, instead of creating a loophole from doing so exploiting an exception?
edit: what do you think is the reason why they made 2 feats specifically for Misty Step and Darkness? I think that these were chosen because they are already granted by some
races, and either the designers believe they are not gamebreaking at 1st-level, or they forgot that variant humans can get these feats at 1st (if it's the latter case, we'll see a level requirement added to the feats when they are published)