For example, if this country had a cheap, fast passenger rail system, people would be more mobile. This would lead to further condensation into startup hubs and there might be more of them. Rents in existing startup hubs would go down, which would be an excellent thing.
Also, universal healthcare. If the U.S. were to join the First World on this issue, it would be immensely valuable to small businesses, and remove a significant component of the risk inherent in starting a company.
I'd also propose this, although it (by definition) has to come from a private source rather than government: an "educational program" targeted toward serial entrepreneurs. The program would be sponsored by a VC firm and provide support, networking, and possibly (but not necessarily) funding. The VC firm would mentor the entrepreneur while he launched startups for a minimum of 12 years. At the end of the program, he would have the option of a job at the VC firm. Thus, entrepreneurship would have "exit options" that are competitive with (or better than) those of investment banking, law, etc.