Add capability to debug DAX code i.e. being able to step through DAX code, add Watch for expressions and variables and most importantly, be able to view the table/DATA change according to the current...
This is crucial. As soon as your model grows and especially when you have to deal with inactive relationships (so you use USERELATIONSHIP or CROSSFILTER) you basically have to "guesswork" what's happening under the hood. Because if you don't use them you can add a table and slicers and see how your filter works, but once you switch contexts and use those, it's impossible...
