One thing I like to throw to players in sandboxy-type games is the possiblity that they are not, in fact, the only group of racially-diverse adventurers out for coin, fame and power. In a setting like Planescape, especially - bashers come from all over the Multiverse, looking for a way to make a quick jink. Or maybe you're a party of monsters (see Kobold Ascension Fight), and the DM wants to show you what it's like from the other side.
And sometimes it's fun to run a "gold rush" style adventure - say, an ancient wizard of a very Lawful persuasion has just passed on, and left his will out to the public, with the location of his tower, filled with many magical artifacts - first to find them, bring them back to the legal offices of the Guvnors, and fill in a specific set of forms, can have them. So now the PCs not only have to deal with getting to the tower, but also the machinations of the other groups interested in the items. Some will travel alone, others will group up for better chances of success - just like the PCs did.
Basics: Cover all the usual "roles" within the party, maybe miss one or two out - after all, while everyone would like to have a Fighter/Mage/Thief/Cleric combo, it doesn't always go that way. It might be a team of Orc Hunters (Fighter/Fighter/Barbarian/Ranger), a cabal of mages (Wizard/Sorcerer/Necromancer/Cleric of the God of Magic), even a Thieves Guild expedition (Rogue/Bard/Illusionist/Expert/Fighter), or any other combo of classes. While it often makes sense, try and avoid the "evil counterpart" trap, of having similar classes to your PCs - everyone will just "pair off", and you'll end up with the mage more concerned about bringing the opposing mage down, not on helping everyone else with their fights. It's also pretty cliche - why would every adventuring party mirror your own team? Make them somewhat different - a coherent theme helps. So "Adventurers in search of Knowledge", "Powerful Wizard and Hirelings", or "Drow Kill-and-Retrieve Team" all add some plot to the group (and make for some funky Party-based Cliches for Risus combats).
Races are harder to pin down - your band of hunters from an Orcish tribe might be all Orcs, or maybe Orcs and Half-Orcs, with Goblins/Hobgoblins in supporting roles, and Ogres/Bugbears in a Heavy Beatstick role. Your Thieves could be Halflings, Humans, Half-elves, Half-Orcs, even something like a Kobold traps specialist or a Dwarven locksmith! It all depends on the racial diversity of your setting - playing old-school games, where ther assumption is that non-humans are rare and mysterious, you'd maybe have one at most in the usual group. In a setting like Planescape, that number grows dramatically - you might even have no human members, depending on the backstory of your group.
So, without further ado - The Ashen Guild, an antagonist adventuring party aimed for use in Planescape.
Desh Stis Irren Khamal, Dust Genasi Fighter/Expert
Seryn Verris, Human Bard
Garrius Destras, Bariaur Barbarian
Rikka Thornfoot, Kobold Rogue
Sark Bloodhand, Hobgoblin Hexblade
The Ashen Guild tend to use many standard adventurer techniques - from a marching order (Sark in the front, Rikka close behind, Desh and Seryn in the middle, and Garrius bringing up the rear), to a party mapper (Desh - her fascination with history makes her pour over older maps, where available, and her own personal maps make great before-and-after shots of the ravages of time), to recruiting Hirelings (standard mercs, maybe a few of unusual races - mainly Extras/Mooks, not much personality).
Rikka is known to reset disabled traps, and, if time permits, might even set up a few of his own - those with skills in trapmaking might even spot his handiwork (think "That's a three-part open ball hinge! Only Kobolds use those..."). He will also improve and alter existing traps (so, while every other trap contains Bloodvine poison, several have Death's Touch paralysing poison on - and expect him to use thestolen Bloodvine needles when you fight him later!). He's also particularly cruel - try and help the PCs get a sense of his dark and twisted sense of humour when they find his traps!
While they don't have a standard Magic-User in the group, Seryn has Bardic Spellcasting - and she'll mostly be packing utility spells (and , in 3.5, both her and Rikka should have pretty decent Use Magic Device ranks), and will use her mining pick in combat instead of magic. Sark is the "blaster" - blending blade and spell, he prepares the proper hurty-spells, frequently augmented by Seryn's spells (so, she can maybe give him something to hurt Incorporeal enemies, or a quick boost of Flight when he needs it).
If the players have any particularly clever plans/techniques, steal and adapt them for the group - they are adventurers, after all, and are just as good as you are!
Cliches for Risus
History-Obsessed Dust Genasi (4)
Master of Many Fields (3)
Student of Several Fighting Styles (Mostly in Theory) (2)
Dark and Brooding Loner (1)
Full-Time Adventurer Archaeologist (4)
Knows Quite a Few Handy Magics (3)
Singer of Songs To Soothe The Soul (2)
Not The Best Choice in Team-mates (1)
Raging Bariaur Death-Merchant (4)
Armour is For Women! (3)
Hard-Drinking Planar Mercenary (2)
Brash and Obnoxious (1)
Life-long Traps Specialist (4)
Sneaky Bastard Kobold (3)
(Easily Bought) Mercenary at Heart (2)
Cruel and Unusual Little Fucker (1)
Sword- and Spell-Slinging Hobgoblin Hexblade (4)
Desperately Searching For A Way Home (3)
Smarter Than He Looks (But Not By Much) (2)
Secretly The Tribe's Chef (1)