ISALL or ISTOTAL DAX Function
There needs to be an "ISALL" or "ISTOTAL" DAX function that would return TRUE if the DAX is being calculated in a total row of a matrix or table visualization, for example, and FALSE otherwise. This would make creating measure totals far easier than using HASONEVALUE OR ISINSCOPE where you need to specify individual columns. This is very important for self-service BI in order to protect users that are not aware of the quirks of DAX measure totals.
Vicky M commented
With Power BI's heavy focus on Self Service BI capabilities, I think this is very necessary, so that we can adapt to a model where data model designers can create measures such that users can fully rely on those to use the accurate totals without worrying about what fields they are using in rows/columns of the matrix table.