There are many long time discussions in this forum about the best way accessing data resides in Dropbox account. The focuses are different. Starting from ability to sync data from different places outside Dropbox folder (even on different drives) to convenient way to keep engaged local storage as compact as possible. Using current Dropbox application implementation leads to "conflicting" situations, requiring sophisticated tricks for workarounds, sometimes. Provided features like "Selective sync" or "Smart sync" aren't always solutions. "Selective sync" ensure local space freeing, but makes unselected folders inaccessible. "Smart sync" keeps data accessibility, but still engage local space when a piece of data have to be accessed. Let's imagine, we should select one from thousands of movie/clip files for something. They are GBs of size every one. We need access to see how starts every one, probably few hundreds of kB up to few MB for every clip, enough to recall what's there. That what Dropbox application does, actually, is downloads entire file (every one) to gives access to just a small piece (the one needed). Is this some example of efficiency?! I don't think so. Many other examples on different types of data could be set. Another example is a user needs to add a file to Dropbox to share a result, for example, using Dropbox context menu. Most users don't expect a second copy to reside locally. How many are forum threads and support tickets related to this? A lot!
Possible solution: Introducing new type of access to account data using virtual block device. Such a solution doesn't engage any local storage (if we ignore a caching - which could be relatively small and optimized space, object of user preferencies) while keeps access to everything inside account. Files, used often (according user preferences, again), could be kept in sync locally (in a similar way like now). Additional preferences for every one folder, in sync locally, where to resides on. Something like current "Selective sync" preferences view, but with path attribute next to every one selected tree entry (not literally same, of course). All supported platforms support virtual block devices to some extend (Linux and Mac fully, Windows partially - missing ability to represent exactly as a folder) - enough for this proposal.
Such move have ability to solve many troubles reported like noted before. Current application organization could be kept available, of course, and every user could select what best match particular needs (i.e. the proposal could be an option).