My gaming group tends to each go in their own direction for character creation. Sometimes this leads to great games where folks are strongly opposed on dramatic elements and character interaction is both very impassioned and very memorable.
Sometimes, the results are less than stellar, and we end up a party like my current Shadowrun group: 3 magicians, and 1 samurai (and until a few weeks ago, no decker or rigger). Or a recent D&D group that had a fighter, a ranger, a paladin, and a wizard.
The players in my group tend to play the same kind of character for a few adventures/games, then switch to a new theme. I've typically been the fighter, but am getting tired of it. I'm getting ready to run Serpent's Skull (at least the first module of it) for Pathfinder, and if the group doesn't have a healer or a rogue, I doubt they will survive long; so I need to nip any potential weakness/blindspot in the bud.
I know I can just tell a player, "nope, too many fighters already" when a character joins an existing party, but what if we are starting a new game? How should I handle a situation where the party either strongly overlaps or has a drastic shortfall in ability? How do I pick between (say) the fighter, paladin, and ranger to recreate their characters to fill the gap? Assuming I get a party of all fighters (for example) should I retool the adventure to require less spell-casting and rogue abilities?