I'm misappropriating the phrase "vision statement" here, which I'm sure (probably) has a well-defined meaning in some domain or another. What I think you guys need to work out is a summary of goals that can be used to separate proposed ideas into "appropriate for Unvanquished" and "not appropriate for Unvanquished" categories, and then (if your goals are specific and clear enough) further separate those ideas from the first category into "good" and "bad".
The first step is deciding what is important to consider. Things like how long matches should last for, how accessible the game should be to new players, or what skills (e.g. manual dexterity versus strategic planning) should be valued most in players.
The second step is getting everyone to compromise on how Unvanquished should treat all the points brought up in step one. For example: an ideal game should last about 20 minutes, accessibility should be valued only where it doesn't degrade sophistication, and the twitch-versus-planning balance should be cleanly separated by a player's chosen role in a team.
It's important to make the distinction between how and what. The examples I just gave are necessarily open to a little interpretation; the point of this exercise is to establish a coherent framework in which specific ideas can be explored to find out what is actually fun. Don't focus on implementation details yet; instead of talking about how "humans should have a rocket launcher", talk about what range a team should prefer to fight at, and how devastating to a team's prospects a "successful" attack should be. You need a strategy before deciding on tactics.
Some things are probably a little too obvious to discuss, but they still need to be agreed upon. Here's my stab at the basics: Unvanquished is an online, competitive, multiplayer, team-based first-person shooter, with two asymmetrical teams and intra-match player progression (of some kind). Also there's building. Those last two need more clarification.
What is worth talking about? I recommend a brainstorming session where everyone lists what they can think of that should maybe be clarified in Unvanquished's vision statement. I'll start with what I can think of at the moment:
- How long should matches last? Is there a minimum and maximum to what is tolerable?
- How accessible should the game be, and to whom (e.g. any person, any gamer, any FPS fan, etc.)?
- What skills should be emphasized (e.g. dexterity, teamwork, strategy)?
- What playstyles should be represented (aggressive offense, turtling, stealth, control, etc.)
- How much preference should the game give to offense versus defense?
- In what ways should the two teams be similar and dissimilar?
- How should the game be paced (e.g. constant action, deliberate and poignant, variable at different times or under different circumstances, etc.)?
- How difficult should it be to recover from a setback (slippery slope)?