I've purposely left the implementation of spells very wide open for referees and players to make their own but I thought I'd share what my original concepts looked like and you can use that as a jumping off point (or not) as you see fit.

Let's take a sample level 3 sorcerer, named Hugas. Hugas fancies themselves a necromancer and has courted a demon patron named Anzu. Hugas find himself on one side of a wide chasm that he needs to cross. They decide to cast a flying spell. To simply rise up on the wind and float is a power far beyond anything the sorcerer has seen or experienced-- that is the stuff of the gods. They do know that birds fly, as do bats. Bats, being often linked to demons, is a good choice. Plus, Hugas has a preserved bat wing in their pouch which can help make a sympathetic link in the spell.

Hugas wishes to channel the power of the bat. This is a second circle spell. It will cost Hugas 2d6 to cast. They use their wand of blackened wood and the preserved bat wing token. This will absorb 2 hit points of the cost. Hugas decides to sacrifice the bat wing to absorb total of three hit points of cost. The total cost will now be 2d6-3, with a minimum of 2 hit points.

Hugas casts their spell. The bat wing is consumed in the casting. They supplicate themselves to Anzu and beg the demon for the power of bat-flight and it is granted. Pain sears Hugas as two bony leathery bat wings grow out of their back, tearing and bursting through their robes. Knowing that the demon is mercurial, Hugas wastes no time in taking flight across the gorge.

* * *

The idea here is that the form a spell will take will be personal to the magician and the entity they wish to call upon. For Hugas, the bat is an excellent choice. For a different caster, it might be an eagle, a vulture, a mosquito, or a butterfly. Or something even more exotic (though that might increase the difficulty of the spell!). The ultimate form the spell takes will likewise depend upon the entity that grants the power. An angelic being might manifest a power as wings made of light. A demon might use shadow or, in this case, a temporary and painful metamorphosis, or they might use a necrotic or negative energy manifestation. How long and what cost, if any, it takes to maintain the spell is left to be set or negotiated. In the case above, Hugas didn't have to pay to maintain the spell but they also know they can't count on how long it would last-- it would suit a demon's humor to watch them plummet!

The other main element, which the above example, does not touch upon is that the magic-user's spells are imitations of those things seen in the natural, supernatural, or beyond worlds. To fly, they must mimic flight. For animal powers, it is sufficient to see them. For things in the supernatural or beyond spheres, they must experience the thing they want to imitate. What ultimate form this takes I leave up to you but, in general, if a magic-user wishes to paralyze people like a ghoul, they must seek out and survive the experience of being paralyzed by a ghoul. A fireball-type spell could take many forms but if the magic-user wishes to cast fire like a dragon's breath, they need to have felt dragon's breath!