Maybe useful, maybe not, but just highlighting I summarised the top 10 suggested FM features across all categories just the other day. It might be a good starting point for some to see what features may not have been created yet. In saying that it excludes everything outside the top 10 so you will still need to search to see if your feature has already been created.
Link here: Top 10 Forza Motorsport Features Fans Want Most - #5 by AntMoose9513