Currently you can sort by collection in the variables explorer which is obviously important however when it comes to assigning variables to elements around your document, you have to find the variable through a drop down that includes every collection.
This quickly becomes a mess if you are utilizing multiple collections and sub-groups. An option to only view by collection would be very useful, similar how you can sort by library for component/style libraries.
There could probably be a lot more done when it comes to organising and managing variables however I think this would be a step in the direction.