To be honest, after beeing through this bad cheating times the first? ranked-pre?-season, having reported tons of obvious botters, just to see them days after again, and nowadays reading about people gotten reported for toxic behaviour or sabotaging teams in ranked tons of times with nothing happening, I still feel writing something in the justification is kinda a waste of time, and stopped doing it a long time ago.
But, they could probably always take cs:go as an example and at least add the most “popular” ones to it, without the need to write anything, beeing self-explanatory (abusive text chat, abusive voice chat, aim hacks, whall hacks, other hacks).
Btw, this is some other point where I lost the believe someone really looks into these reports. If some support-person really would be working with this date, at least this person would be pressuring colleagues to put more options into it. And can it really be this complicated adding a few more options…