Create a central resource for all agency web teams to communicate, share code, establish conventions, and learn from one another. Then put a premium on making agency systems interoperable (probably through APIs because unification on a single platform is a pipe-dream).
With agencies working together to establish conventions, the building and maintenance of federal websites would be far more efficient, enabling a reduction in cost and/or allowing for more time to be spent on important stuff like making sure citizens can quickly and easily access the information they need.
Bonus Points: Make the resource public so the folks who pay the bills (i.e. “we the people”) can benefit (as can state agencies, which are often in worse shape than federal ones).