Perhaps it would be easier if I gave some examples:
From 3.0 Magic of Faerun:
I don't think there's any existing official rules. Some off-the-top-of-my-head suggestions though
Natural locations:
Mystic Maelstroms - Use the Wild Magic surge table for any spells cast in the area. If it's strong enough of an area, those in the area might need to make Wisdom saves or suffer an effect from the surge table
Sparks - Lightning spells gain advantage on damage, saves against lightning spells are at disadvantage.
Fey Mounds - Can increase the spell level by 1 for Enchantment spells cast in the area. Objects left overnight in the area might pick up a magical effect that lasts for a day (weapons treated as magical, armor glows with Faerie Fire, wine becomes a healing potion, a gem stores an enchantment spell of a level per 100 gp value, etc.)
Boomshroom patches - Athletics or Acrobatics check (DC 10?) to walk through the area or suffer 1d10 (or more) damage. Check with disadvantage if dashing.
Doom Pits - Falling in deals 1d10 (or more) Necrotic damage, Constitution save for half.
Magically enhanced sites:
Crossroads - undead cannot leave the area unless they make a successful Wisdom save. After dark, Conjuration magic cast at the crossroad gets a free one-level boost or targets save with disadvantage.
Backroads - After dark, illusion spells get a free one-level boost or targets save with disadvantage.
Mythals - Spells cast a mythal have double duration (if not instantaneous) and Arcane spells do not need components.
Druidic Circles - Druid spells do not need components and spells that affect beasts or plants have maximum effect or grant advantage/disadvantage on saves (caster's choice)
Monastaries - As an action that takes 1 minute, monks can regain 1d4 ki. This can be performed once per short rest.
Shrines - As an action that takes 1 minute, clerics and paladins can regain 1d4 spell level slots. This can be performed once per short rest.
