Here are a list of ideas that I have for the above classes!
Alchemist
Alchemist has some difficulty closing out the game when it comes to end game (n7+) due to the wildcard nature of it. While this is somewhat fine it does lead to the Alchemist being difficult to reach their win condition! One suggestion I have for this is making it so that the day ability - the potion that makes you immune at night - also makes you appear Not Suspicious when used!
With a bit of luck and skill you could potentially reach the point in the game to finish out your win condition which I believe is a positive thing and most abilities already in the game have similar elements.
Reaper
Neutral killers have a lot of trouble honestly closing out the game at all skill levels, deducing who is lying about their role becomes easier and easier as the time passes and without allies to back them up neutral killers end up getting that axe all too soon and fall short of their goal.
The abilities of the reaper are very strong - as a class themselves they are a force to be feared. The issue doesn’t lie with their abilities in my opinion but rather the win condition - it is essentially somehow live until the end of the game. My suggestion is simple and goes with the story and the character of the reaper! Instead of kill everyone else the reaper instead wins the game when they collect a certain, set # of souls.
There’s a lot of nuance to this and some abilities might have to be reworked but the thought of a killer who doesn’t care what his targets are and just killing randomly while creating chaos I think is a fairly good and intended aspect of the neutral killer role and allowing for more craziness after the win condition has been met.