There's always going to be that one more thing to fix, but if you fix everything, what will be left of you?
Are there any mistakes in that sentence? It doesn't seem that bad, but I don't know if a semicolon or quotation marks or something would make it better. Any suggestions?
It looks OK to me, I don't know about having '...to be that one more thing', to me it sounds better just as 'to be one more thing'.
Welcome to the site. In my experience it's more idiomatic to say there will (sometimes contracted to there'll), instead of there's at the beginning, as the sentence conveys a definite sense of future that is better expressed by there will than by there is.
There's not anything really wrong with the comma after fix, but my personal style is to make that comma a semicolon, to better show the separation of the two clauses and thereby reducing the chance that a reader might be temporarily confused about where the boundary between the two clauses is.
The choice of of you as the final prepositional phrase is interesting because it conveys that a fixer always leaves something of him- or herself with every fix, meaning that a fixer can become "used up", with less left of him or her after the fix than before, while changing that phrase to to you conveys the notion that there is nothing left for the fixer to do, so the fixer no longer has purpose.
If the sense is that the fixer will have lost his reason for existence, I'd say "... what will become of you?"
If the sense is that the fixer will have lost his occupation, I'd say "... what will left for you [to do]?"
Punctuation aside for a moment, as Mr. Ashworth has noted in comment, the construction is illogical. If there's always going to be that one more thing to fix, then you never can fix everything and you'll always have something to do.
How does this sound?
There's always going to be something that needs fixing. But even if you could fix everything, then what would be left for you to do?
Answer: screw something up and get busy.
