My 2cp...
I've always been willing to let gods step in as patrons, but I strictly limit it to deities that have magic, or knowledge, or something similar listed as part of their folio. I checked my copy of the 1st edition Deities and Demigods, and it seems to me that ol'squiddy face could work for either wizard or cleric character. He's listed as both a 16HD warrior and 20HD illusionist, other powers include summoning minions, regeneration, teleportation, and causing insanity the anyone within 100miles who fails their willpower save; at least when he's awake. It also mentions that he has really powerful spell resistance and is immune to cold, water, vacuum effects and that he will flee if confronted with certain powerful elder sigils. It is worth noting that normal animals and other creatures of the Prime Material Plane instinctively know that the cleric or wizard is tainted and will always react negatively to that character; attacking or fleeing... so at the very least, the character must walk or ride in someone's wagon whenever they travel and would impose a skill penalty for the rest of the party when dealing with their horses, familiars, etc.
Using this as a template, I'd populate the magic user's list of potential spells with anything that relates to these powers and it would look something like this:
1st-
Charm person,
Chill Touch,
Detect Magic,
Invoke Patron (Cthulhu) - which would be a very dangerous spell and probably send aid in the form of shared immunities, summoned Deep Ones, regeneration for the caster, magic resistance, possibly even grant a gaze attack that causes insanity for the victim, that sort of thing.
Runic Alphabet (mortal),
Sleep,
Ward Portal
2nd-
Arcane Affinity (Chaos),
Levitate,
Locate Object,
Mirror Image,
Monster Summoning - limited to minions of the elder lord and other mutants and abberations,
Phantasm,
Ray of Enfeeblement,
Scare
3rd-
Consult spirit... if the wizard is truly brave or foolish enough to try...
Eldritch Hound,
Emirikol's Entropic Maelstrom,
Fly,
Make Potion,
Paralysis,
Planar Step,
Water Breathing
4th-
Control Ice,
Polymorph
5th-
Lokerimon's Unerring Hunter,
Magic Bulwork,
Mind Purge
As a chaotic patron and character combo, the wizard would be able to cast any spells they gain access to, but the list above covers what could be learned from studying the lore of the elder gods or powerful invoke patron result. The wizard would gain these spells through elder minions and from dreams... both of which would definitely lead to insanity for the character.
As a cleric the character would have much less casting ability, but closer contact with the mind of their god and would end up with powers based upon their deity's powers and abilities as they advanced in level; aura of fear, regeneration, etc., and would have to be played as stark raving lunatics. I would let them have healing touch, but it would force the recipient to roll for corruption/ mutation, and they would be able to
turn/ destroy normal creatures and humanoids. However, I would probably double their chances for deity disapproval and give them all sorts of little stigmas and taboos, like turning them into soul-sucking cannibals and other such things to make life horrible...
1st-
Darkness,
Detect Magic,
Paralysis,
Protection from (Law),
Resist Cold,
Second Sight
2nd-
Banish,
Curse,
Divine Symbol,
Lotus Stare,
Snake Charm,
Stinging Stone
3rd-
Bolt from the Blue,
True Name,
all fourth level spells...
5th-
Weather Control