In 3.9, I started taking a look at solving some long-standing scaling issues with dropdowns, notably those for users (#19867) and pages (#9864). I arrived at a conclusion about our mixed usage of autocomplete and autosuggest far too late to make it for 3.9, and did not add it to my plate for 4.0, but would like to tackle it again for 4.1.
We can solve these issues smartly by using a combination of Ajax-powered autocomplete/suggest and smart defaults, e.g. users with the most content pre-loaded for quick access without having to run a search. As a brief overview of where I see us going with this – a roadmap, if you will: user dropdown, page dropdown, current instances of jQuery UI autocomplete (multisite users and new site), tags/nonhierarchical taxonomy UI, more built-in taxonomy UIs (#14877), and possibly more as we discover appropriate places. Not all of this will happen in 4.1 – think of this as not only a solution to long-standing, painful problems, but also a step in a series of many.
To that end, we will be holding an initial chat at 19:00 UTC on 2014/10/8 to get things moving. @ericlewis and I have begun early development as a plugin – for more, see this particular branch, which experiments with using Select 2 as a JS helper library for the UI.