Without changing the mechanics of the game a real set of symbols will be difficult. But why actually draw the symbols? Just tell the players that there are symbols...which much like words might have some common roots but have radically shifted in various times and palces so that they are barely recognizable as the same thing. Take the name "John" for example: Jon, Johan, Ivan, Van, Ian...and so on. Anything that transmists meaning changes with use, and given the number of races and spellcasters in a typical fantasy world that is bound to happen.
So what are you left with? Encourage spellcasters to describe what they are doing. If a spell has a somantic component then the spell caster says "I draw the eldritch sign for Flaming Sphere in the air, and cast my material components through it as I shout the magic words in the arcane tounge!" If no verbal component, or if the caster has Still Spell, then it might be something like: "In my mind I overlay the symbol for the Stilled Bull's Strength on the Fighter and whisper the words as I offer the hairs of the Bulls of Keltos back to the world." If someone uses spellcraft to identify a spell you might say on a succesful check: "While the symbol is not exactly the one you would use, it obviously has the core elements of Sign of Expeditious Retreat in it."
There, the flavor you wanted, w/o all the problems of trying to create a systematic (and thus nearly impossible with RAW) method for representing the various spells.