The basic requirement here is that modules are able to present their own UI in such a way that together, DSpace modules' UIs feel like a single application UI as opposed to lots of separate apps. Internationalisation is also important, as well as localisations ('skinning' the site or adapting it for a particular organisation).
wiki:I18nSupport Internationalisation support has already been started.
In terms of a UI framework that better supports modularity and customisability, Texas A&M university in collaboration with others are developing an XML UI framework built on Cocoon called Manakin.