In all fairness, the Overview has only gotten better with the many changes introduced in Trinity (after a bit of bug squashing), and most of the changes one can wish for now only exist as a result of this new wealth of content becoming harder to manage.
This is especially true for the many players who multi-box or don't always run EVE from the same computer (although the /LUA and a good USB key with a RAM disk can go a long way, but that's another issue).
The obvious thing one can wish for is a simple way to import/export and store overview settings, since they're client-side already… xml comes to mind, even though a simple txt file a-la prefs.ini would do just as well.
That would solve the CTS-inducing chore of redoing one's entire set of overview settings for every alt in the book (that can be a lot), every time we switch computers or the EVE install goes to hell for any reason — not to mention the life-changing ability to share overview presets with corpmates and friends, and make sure nobody forgot to add effect beacons to their brackets layer. ;)
Another feature that would ease things quite a bit would be the ability to select/deselect an entire category with a single click of a checkbox next to the Category name in the Overview Settings window (Asteroids: Off ; Entities: Off… \o/).
The next logical step is for the in-game overview editor to sport 'Favorites' custom Overview categories, which one could use as blocks to build Overview presets.
Custom Categories would be created and (re)named by the player, and would allow one to add any overview-friendly Type by a right-click (either from the HUD or from the Overview Settings 'Types' list) to a new or existing Custom Category (even if said Type is already present in one or more Custom Categories already).
Building a new Overview Preset could then become a simple matter of ticking the checkboxes next to whichever Custom Categories' 'folder' one wants to add to the preset. …bliss.
I only meant to post a much simpler feature suggestion, also overview-related, but got carried away and covered the whole Overview issue, which doesn't make for a wall of text, in the end… so here it is:
Add bookmarks as a Type, under the Celestials category, so that the same bookmarks that can be seen in System Map can also be displayed in the Overview list and/or as brackets.
Tacklers, CovOps and snipers will love you long time for this one, and I suspect it's a 5' job for the interface team. ;)