What should my business proposal say?
And putting together a proposal is obviously about bringing together ideas, people and capital.
The proposal should sell your business idea to a potential investor from all three areas. The idea, why it's a great one, including needs to how you will deliver. The people and why an investor would back you and your team. And the money, how are you going to give the investors/bankers a return.
A great book on this is by Keith Cunnigham called Keys to the Vault.