Hey there,
I'm not studying Robert Frost, but from my ten minutes of background reading on the poem and analysis, you could talk about walls as a motif throughout the poem (considering how the name of the poem is 'Mending Wall') or symbolism- how wall-building could be interpreted as a way to self-confine a person or to assert their stance (e.g. how walls are used to assert what's considered as 'their property').
Remember, these are my initial thoughts since I'm not studying Robert Frost, but I hope this helps!
Definitely some solid ideas here - and yes it can be an extended metaphor/motif. Walls are extremely symbolic of barriers, either self imposed, or constructed between people over time, cultural barriers, so many! Walls have been used in music, poetry, art etc, so is a common symbol.
I would say in this circumstance that the persona is really wondering why they need the wall at all (considering he has apple trees and the other has pine trees or something like that?), and is also looking at the self imposed barriers as a result of preconceived perceptions.
From your explanation, it looks like you are narrowing it down too much, and looking at the meaning just in that phrase, rather than the whole poem (I could be wrong) - try 'zoom out' a bit, and say 'Frost explores the self imposed barriers that the characters establish through the extended metaphor, evident in "insert quote". This phrase is also symbolic of wider society, and how preconceived opinions shape our values of our surroundings". The sentence structure is terrible there, but hopefully you get the idea
Please reply if it doesn't make sense!