Primary concern would be viability, ofc. Second concern would be to fill all essential party roles. Points for parties with an interesting theme.
This is hardly a challenge. This gives you some of the most versatile classes in the game.
Clerics, oracles, bards, inquisitors, druids, magi, and summoners are all d8 classes, and they all have well known melee builds while leaving plenty of room for spell use.
Lets see.... How about A Sarenrae focused party?
Dawnflower dervish bard (highest attack bonus of this group, decent enough damage with arcane strike), cleric and inquisitors with scimitars... hmm, how about a life mystery oracle (since you can say it was gifted by sarenrae. Yeah, this can work well. The fact that two of the classes have summon monster spells on their lists should helps make them harder to take down.
Summoner/Druid/False Priest Sage Sorcerer/Shaman = effortlessly annihilate any AP at any level of play.
Summoner and Druid are both (effectively) frontliners. The Druid's Companion will lose out to Eidolon for awhile, but eventually the Druid will get Wildshape and be able insta-gib people in a single bite.
False Priest Sage Sorcerer - All of the free spells. All of them. Handles INT based skills and trapfinding thanks to Aram-Zey's Focus.
Shaman - Mugs spell lists for what is needed at the moment, hooks the Sorcerer up with divine scrolls of Arcane Spells, hexes all day.
The 6 level caster power hour (plus Druid) covers most challenges.
Summoner, Inquisitor, Druid, Bard.
You lose out on some stuff, but make for a fun party, and effectively have 6 party members so that helps. And all buffed to the gills between the Bard and Summoner, while the Inquisitor buffs himself to heights that make the Fighter wet himself and cry. Make the Inquisitor a Sanctifed Slayer and he loses out on some of the Judgement flexibility, but can get Trapfinding or omething if the party needs it.