Ok here goes:
Hey. Just wanted to introduce myself. Lurk around here but don't really post. I play pubs only, and follow comp TF2 sometimes, namely 6's. Got exicited reading this thread because Valve is actually trying to do something for this community - even if it isn't 6's, it's a great place to start. A person like me would definitely be interested in this.
I read the OP and can summarize into these bits:
1. Valve is willing to implement an in-game lobby system
2. They want 9 player teams, each class plays
3. They want players to use unlocks/sidegrades
4. They want a weapon pick/ban system
These things are a must. Without them the system won't run. So considering these limits, I thought about this for a while and have a major suggestion of my own now.
I don't know if you're reading this Robin/TF2 Team, but I think you've already created a similar system that works. If you made a few changes to that system, an in-game lobby system could work really well.
I think MvM is the best example to demonstrate this. Imagine an in-game lobby like MvM - players can join a team, they work together towards the goal, they are rewarded for it, and can "unlock" better abilities and weapons for the next round. Imagine if this were a lobby instead focused on high level play:
1. You have 2 teams of 9 players joining.
2. Every first round starts with stock (fair and will encourage stock use)
3. Players obtain "group points" for their team based on actions like kills/ubers/headshots/sapping/building, etc.
4. After the first round ends, the team group points allow each individual player to "unlock" a weapon/(s) of their choosing.
5. The more powerful ("op?") a weapon is, the harder it is to unlock, so essentially a team is rewarded for playing well while others will have to adapt <non-stagnant gameplay
6. Since it's beta, at the end there could be a voting session on items and whether or not they should have stats changed.
This idea has most of your wishes in it, and opens other opportunities (you can cash in on lobby items like strange parts and awards, achievements, etc). You can add more maps/arenas. There's a lot of starting ground already without having to alter much. You could make changes as you collect data as well.
Some major concerns:
This team point system - make it so every class can shine. There are power classes in TF2 who take all the glory - an ubered heavy combined with a medic will always be more powerful than a lone player. Because of this, most comp games will center around them medic/heavy/soldier and everyone else just goes to the sidelines. This might be causing the stale strats. You don't want that. And you want people to use more items.
Then reward the classes equally - medic kills with assists should always be worth less than kills done without a medic. Sapping a well placed sentry takes effort, reward the spy, building uber without dying should be bonus points, airshots - each specialty should be judged fairly and rewarded fairly so ALL classes feel like they've done the same effort without just having a medic uber them and getting easy kills. Now add in sidegrades. This would introduce different styles of gameplay and encourage different form of gameplay.
Looking at things in the future, this revised point system for lobby could work, but people might abuse it as well. Teams would become too focused to get points. But perhaps you could make it so upgrades would contribute less points or something, I'm sure it wouldn't be too hard to create a good system of balance.
Nothing here is absolute. I just wanted to give a good starting ground on something that I could immediately visualize for this game. I enjoy playing TF2 a lot and this and maybe any other lobby in-gamesystem would be better. Thanks for taking the time to read this and I hope it comes into the making soon!