I'm posting this after renting and playing Halo Reach for 360. I loved the campaign but, the multiplayer was disappointing. It made me long for the old days of playing online PC multiplayer shooters like Starsiege Tribes and the first Unreal Tournament. I remember the dedicated servers, the mods, the variety and amount of different maps, etc.
Those are the things that indies need to do. The big developers and publishers don't like to let people set up servers anymore or make mods. They say things like "We wanted to make it a seemless experience." ,"We want to make it easy for the player to jump right into the action.". Guess what. FPS players at the core are hardcore gamers. We want to be able to mod maps, weapons, gameplay, or at least want to be able to pick what friggin maps we want to be cycled. There are a few exceptions with the big guys. Valve still seems to understand what makes PC gaming great.
What I want to see an indie studio do is. Get a good game engine like Torque 3D or Unity and make a solid multiplayer FPS. Make 3 high quality maps doing a team deathmatch or CTF gametype. Start off with 3 or 4 weapons that you balance really well. Iterate from there. On the map side have designated sections that a team spawns at. Nothing is more annoying than seeing a guy from the opposite team spawn right behind your base. Try your best to randomize some spawning within the areas and have a short invincibility when spawning, to discourage spawn kills.
Obviously this is an easy concept. The hard part is the execution. Don't go trying to compete with Halo campaign. They have too many resources with art and budget for you to hope to compete. I know in my heart that there is a good chunk of people who would buy a hardcore FPS Multiplayer only game on the PC and Mac.
Take the concept Valve's Team Fortress and break it down into a smaller project that you as a team of 4 or 5 can accomplish. Don't feel like you need to have every idea you have in the initial release of the game or that it needs to be perfectly balanced. If it's fun people will put up with some minor annoyances especially if you show that you are commited to iterating the product. Valve constantly does updates and fixes and still have a ton of players playing.
I hope this gives people some encouragement to make this kind of game. I'm so bored a lot of what is being put out there, especially on console. I hope I get to see some things in the future.