So you mean that it'll be a pane only for data frames? Environment already has a distinction between data and values. Do you think it's not fine-grained enough for you?
One more trick you can do is to use F2 to open saved data frame in the Viewer. For example, if you ran the code that creates data frame named df, you can then put your cursor on df, press F2 and it'll open this data frame in the viewer. This way you don't even need to look in environment pane to find data frame that you need.