Unsolicited proposals differ from solicited proposals in an important respect: Your audience may not be aware of the problem you are addressing, so your proposal must first convince readers that a problem or an opportunity exists before convincing them you can address it. Thus, unsolicited proposals generally spend considerable time explaining why readers should take action and convincing them of the benefits of doing so.
With virtually any proposal, keep in mind that you are always competing for something—money, time, management attention, and so on. Even if yours is the only proposal on the table, you are still competing with all the other choices your audience members could make with their time, money, and attention.