The federal government's primary role should be enforcing only the most universal of rights. As a secondary role, it should enforce interstate treaties. For example, I'd suggest that universal healthcare should be a state issue, and if larger than single state collective bargaining is desired, then one could enact interstate treaties for the purposes of bargaining. (eg We hearby create the following Organization which will bargain on behalf of the Member States for the following Services...)

Infrastructure

Basic infrastructure must be under government control. For example, we grant that all people should have water, and it therefore is to be controlled by the government to ensure that all people can get water. The same goes for electrical power. The same goes for roads (travel). The same goes for physical lines associated with data transfer (read: phone lines). The same goes for housing.

Government (local?) should provide a basic level of service for a variety of needs. For example, there should be a minimum proscribed level of education provided by the state at a particular price (though consumers are free to choose to spend their education money elsewhere). There should be some level of food (rice, beans, cured meat, certain vegetables) provided at a particular price (though consumers can buy other food elsewhere, prepared or otherwise).

Based on basic levels of optional infrastructure service mentioned above, (some infrastructure such as roads, phone lines, power lines, etc, can't logically be optional. Either they are provided, or they are not, and you can't reasonalby have some company come by and tear up the roads in front of your house to replace them with better roads.) the government should provide all people with sufficient funds to purchase the minimum levels of goods. In other words, if the government charges $400 for enough food to keep a person healthy for one month, than it must give citizens $400 every month to buy food. It should not require citizens to buy its food, only provide them with the opportunity to do so, since the point is to replace welfare with something more assured. Before anyone points out the so-called inefficiency of government, lets remember that when the DoD buys a $700 screwdriver, it is some private company that is selling it to them at that price.

Enforcement

Given that the government has some purpose, it must also have some means of carrying out that purpose, which we shall call Enforcement. The standard method is to have several departments within the exective branch which carry out enforcement, such as Investigations, Marshalls, Army, Navy, and so on. One alternative is to always expect the Member States to handle enforcement, via National Guard, Police, or some other means.