Runes would add a little more character to the symbols, but they would remain as the OP suggested (distract from immersion). The aura idea is more subtle, but might be a tad annoying as all the NPCs would have this glow about them, and different lighting may make the aura look different in different areas. A hotkey activated toggle still seems to be the best of both worlds, although, again, I'm not bothered by the symbols.
