There’s a nice idea I’ve seen in Trudvang, where magic has to be pulled toward the caster. In essence this means that elsewhere the magic becomes thinner. Think of the Faerunian ‘weave’ as actual fabric. Casting spells pulls on that fabric and it gets rouched up in places.
The Trudvang idea is best served by example. Say your wizard casts Fireball (I know, what are the chances?). She’s pulling on the magic ‘thread’ to bring fire to her location; so somewhere else, fire is reduced to feed her spell. Perhaps the fireplaces of a farmstead gutter and go out, or even a dungeon-delving party find their torches go out and won’t relight for an hour, with no explanation.
There’s plenty of magic to go round, so the creases in the magic weave tend to get ironed out (unless, say, there’s an almighty mage war), but it might help give some colour/things to add to random encounters.
On a more mundane level, for smaller spells, it could be that effects are drawn from the immediate vicinity - a little like the Wild Magic idea but very much based on the spell just cast. Eg cast Magic Missile, and your archer loses an arrow; cast Fly and the Cleric’s hair blows like it’s in an 80s rock ballad video for ten minutes; cast Shield and someone else in the party loses 2AC for a round. I dunno. An idea. Would make wizards be looked at askance.
Clerical magic would obey different rules, as clerical (Druidic, Paladine) magic creates rather than draws from an existing resource - hence why Cure Wounds doesn’t make some poor turnip farmer die in his field from an apparent sword cut.
Or...does it? Would definitely make ALL magic be viewed as dangerous and “not welcome ‘ere”...