A document to synch up vision and goals between partners doesn't look very much like the standard business plan (or what's been proposed so far here).
With partners you should have a mutual understanding of:
- Risk tolerance. How much of their own money will they spend? How long will they go without a paycheck? How cheaply can they live?
- Contribution. How much time/money will each partner contribute?
- Equity. Who owns what? I'm a huge proponent of getting equal partners. If they don't deserve an equal share, punt 'em and find someone who does.
- Vesting. How long will it take before everyone owns their share (should be 2-3 years - you don't want someone quitting early and leaving with 33% of your companies stock)
- Goals. Do you want to build a happy little income producing side project? Or a monster business? Or do you want to change the world?
- How to leave, gracefully. There should be a written agreement that has rules for when a partner leaves voluntarily, or when two partners want to push out a third. It's a lot easier to write this down NOW than when your company is worth $10m.
- What you're building, who you're building it for, and how flexible you are on the idea. Ideas change, a lot. You should be passionate about the same things.
- Authority. Does anyone have it on design decisions? Code decisions? fundraising decisions?
- Horizon. How long are you committing to the project before you punt it or pivot to a different idea?
In terms of business plans, it's a great exercise to discuss stuff like development process, roles, sales/marketing, pricing, etc. But don't spend any more time writing the document than you HAVE to to get a shared understanding. 99% of the value is the process-- they usually collect dust once they are written.