I use custom SQL quite a bit in both Tableau Desktop and Tableau Prep, and in both cases I feel that the Edit Custom SQL functionality is an afterthought. In any other programs I've done custom SQL, there are formatting helpers that make a huge difference in the speed and quality of my code. Simple things like a different color for table names, functions, or strings would make both writing and understanding existing custom SQL much easier. Even something similar to the formatting you get in calculated fields would be helpful (autocomplete, custom colors, etc.).