system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] How would a Boggart portray a more abstract fear?

A Boggart's abilities seem to be limited to shape-shifting itself, it doesn't seem to be able to affect the senses of the person facing it.

How then would a boggart deal with someone with a fear that isn't as cut and dry as 'spiders' or 'dementors'? What about a fear of  being unimportant, or the dark, or being lost, etc?
gpt
Boggarts transform into the moon in from of Lupin, but Lupin isn't exactly afraid of the moon- he's afraid of transforming into a werewolf and losing control. It's safe to say the would probably do something similar in your cases- using symbols or triggers to reflect the vaguer fear.