Stuff I've played with d20 Modern:
- A military and scientific team working together on a mysterious island
- A special investigative unit of the Bureau of Indian Affairs that checks out paranormal crimes a la X-Files
- A group of bad-mutha martial artists who are brought to an isolated island by a megalomaniacal bad guy to compete in death matches
Generally speaking, there are two big ways and one little way in which you can get a group together:
This is our job: On one level or another, the PCs want to do this, trained to do this, and are meant to do this. They can all be some sort of soldier, some sort of cop, whatever. This way makes motivation easy but flexibility hard. You can have a guitar-playing psychologist on your Marine Corps zombie-killing unit, but he probably isn't going to fare quite as well as everyone else... and it might be a stretch to work him in.
D'oh: People are thrown together by chance (plot), fate (plot), or circumstances (plot), and have to work together, even though they come from different backgrounds. Ordinary people who suddenly find zombies wandering through their town, or a bunch of people in a coffee shop when the coffee shop swirls through a dimensional nexus and comes out in fairyland. Whatever. This makes flexibility easy (Dedicated/Charismatic with no combat feats? Sure!), but motivation is a bit harder, unless the GM shoehorns people into things. This doesn't mean that the GM is bad -- in this scenario, it's the GM's job to mostly force the team together at first. The drawback to coming together this way is that some people are going to try to game the system. "Yes, Marine Martial Arts Specialist Hendricks was in Starbucks, getting his mocha, and he just happened to have 12 tear-gas grenades, night vision goggles, and his Mastercraft +3 leather jacket with him when that dimensional vortex hit... gosh, is this going to be a combat-focused game?"
And the little way:
Not working together: This one is sort of for political campaigns, campaigns where you've got very experienced players who are really comfortable getting nasty in-character and keeping it friendly but competitive. Sort of think "Babylon 5", if people got to play Sheridan, Londo, and G'Kar.
Let's take a scenario in which a fictional city begins to have weird crimes, and multiple groups slowly realize that the Fey have entered the world, and pixies and elves and unicorns are doing weird stuff:
- An archaeologist wants to track them down to figure out what they're doing here, what dark purpose they serve.
- A cop wants to get these new monsters off the streets by whatever means necessary.
- A local crime boss wants to get these things off his turf, or at least come to an alliance that gets them out of his bidness.
- A District Attorney wants to get the truth out there, get the Fey to be treated like anybody else, which means not just killing them on sight, but fitting them into his idea of order.
These folks don't really need to work together. Sometimes they'll be joining forces -- the D.A. and the Cop stopping the Fey from kidnapping children and putting the humans who helped them behind bars, the Crime Boss and the Cop gunning down a dangerous Fey monster, the D.A. and the Archaeologist trying to figure out where they're coming from with street smarts and arcane lore.
And sometimes, the Crimelord will be trying to gun down the Cop, who's trying to shake down the Archaeologist for information but being threatened with an Internal Affairs investigation by the D.A, who is also trying to figure out whether to use the Crimelord as a scapegoat in the crimes the Fey have been committing...
You can do other variations -- like "The group that is not really suited for the adventure but has some other reason to be there" (for example, a travelling a cappella group that keeps running into attacks by the undead, who, for some reason, really really hate a cappella) -- but I think most stuff can go into one of those three categories.