Have Data.gov serve as a platform for serving national data. In addition to serving as a catalog, provide actual data hosting capability, e.g. hosting national framework data layers to support a wide variety of mapping (ala The National Map).
Additionally, this may be of particular value to support collaborative and cross-agency efforts such as Imagery For The Nation – Imagery For The Nation (IFTN) is an attempt to collaborate on acquisition of orthophotography, with an eventual idea of seamless coverage for the nation. There are similar ideas regarding other base layers, e.g. transportation, hydrology, cadastral and so on.
Hosting may provide an easier means to agencies for providing public access to data given internal firewalls, infrastructure and/or budgetary constraints. Data could be hosted for read-only download (FTP and HTTP access) via a cloud-based provider or scalable datacenter, with secure upload accounts provisioned to authenticated data stewards.