(before coldburn berates me for trying to give clerics even more fun/power at the expense of other classes, I'm also including wizard here though they have the conjuration sand thingo/book thingo already, which druids/clerics have too but makes more sense for wizards/sorcs)
How it would work is check your domains, and then pick one or the other to summon a reprentative from when you use summon creature I-IX.
For wizards with a speciality use the following: (or sorcerors with a school focus) For Evocations: Choose one at random of Air, Water, Earth, Fire, Destruction For Abjurers choose: Protection, Good, Magic For Transmuters choose: Strength, Healing, Plant For Illusionists choose: Trickery, Travel For Necromancers Choose: Death, Evil For Conjurers choose ANY one at random. For Diviners choose: Knowledge, Sun For Enchanters Choose: Luck, War
Air: Air elementals, Lightning elementals Animal: As per normal. Death: Shadow Fiend Type things, Visages etc Destruction: Sonic Elemental Earth: Earth Elemental Evil: Negative energy creatures, undead Fire: Fire Elementals, Smoke Elementals Good: Positive energy elemental, good fey creatures Healing: Acolyte of healing Knowledge: Magic: Quasit, Animated weapon Luck: Jester, Clown, Wild magic elemental Plant: Twig Creatures, Ooze, Shambling mound Protection: Shield Wielder Strength: Minor Golem Sun: Radiance Elemental Travel: Flying Creatures, Blink Dog Trickery: Doppleganger, Invisible Stalker Water: Sahuagin, Water Elementals War: Reaver (2-weapon wielding beast/demon)
Just an example anyway, you'd need to look throgh the creature lists and see whats there and balance them for each summon creature spell I-IX. It is a lot of looking up and making tables though so just a possibilty :)