Or rebuild anything. Have the option of sending allied settlements as garbagemen and cleaning up Sanctuary. Or establish guard posts along the supply lines. Or have more story-based content with the settlements/Minutemen.
Domesticating the native Mirelurks as the region's "horses". That's be cool.
Also, why aren't there carriages in Skyrim? That seems like a wasted opportunity for both Fallout and TES alike.